public static float Determinant2x2(float a, float b, float c, float d)
    {
        return a * d - b * c;
    }

    public static float Determinant2x2(float[,] i2xj2)
    {
        var m = i2xj2;
        return Determinant2x2(
            m[0, 0], m[0, 1],
            m[1, 0], m[1, 1]);
    }

    public static float Determinant3x3(float a11, float a12, float a13, float a21, float a22, float a23, float a31,
        float a32, float a33)
    {
        return a11*Determinant2x2(a22, a23, a32, a33)
             - a12*Determinant2x2(a21, a23, a31, a33)
             + a13*Determinant2x2(a21, a22, a31, a32);
    }

    public static float Determinant3x3(float[,] i3xj3)
    {
        var m = i3xj3;
        return Determinant3x3(
            m[0, 0], m[0, 1], m[0, 2],
            m[1, 0], m[1, 1], m[1, 2],
            m[2, 0], m[2, 1], m[2, 2]);
    }

public static bool Straight(Vector3 a1, Vector3 a2, Vector3 b1, Vector3 b2)
{
    var a = (a2 - a1);
    var b = (b2 - b1);
    var c = new Vector3(a.x/b.x, a.y/b.y, a.z/b.z);
    return c.x.EqualsApprox(c.y) && c.y.EqualsApprox(c.z);
}

двойное скалярное и двойное векторное произведение, а не тройное

public static bool Straight(Vector3 a1, Vector3 a2, Vector3 b1, Vector3 b2, out Vector3 result, out float t)
{
    var L1StartPoint = a1;
    var L2StartPoint = b1;
    var L1Direction = (a2 - a1).normalized;
    var L2Direction = (b2 - b1).normalized;
        
    result = new Vector3();
    t = -1;
        
    float A11 = L1Direction.x;
    float A12 = -L2Direction.x;
    float A21 = L1Direction.y;
    float A22 = -L2Direction.y;
    float B1 = L2StartPoint.x - L1StartPoint.x;
    float B2 = L2StartPoint.y - L1StartPoint.y;
    float t1, t2;
    float det = A11 * A22 - A12 * A21;
    if (Mathf.Abs(det) >= Mathf.Epsilon) // if det != 0
    {
        float d1 = B1 * A22 - A12 * B2;
        float d2 = A11 * B2 - A21 * B1;
        t1 = d1 / det;
        t2 = d2 / det;
        float temp1 = L1StartPoint.z + L1Direction.z * t1;
        float temp2 = L2StartPoint.z + L2Direction.z * t2;
        if (Mathf.Abs(temp1 - temp2) < Mathf.Epsilon)
        {
            result = new Vector3
            {
                x = L1StartPoint.x + L1Direction.x * t1,
                y = L1StartPoint.y + L1Direction.y * t1,
                z = L1StartPoint.z + L1Direction.z * t1
            };
            t = t1;
            return true; // if temp1 = temp2
        }
        return false;
    }
    A21 = L1Direction.z;
    A22 = -L2Direction.z;
    B2 = L2StartPoint.z - L1StartPoint.z;
    det = A11 * A22 - A12 * A21;
    if (Mathf.Abs(det) >= Mathf.Epsilon)
    {
        float d1 = B1 * A22 - A12 * B2;
        float d2 = A11 * B2 - A21 * B1;
        t1 = d1 / det;
        t2 = d2 / det;
        float temp1 = L1StartPoint.y + L1Direction.y * t1;
        float temp2 = L2StartPoint.y + L2Direction.y * t2;
        if (Mathf.Abs(temp1 - temp2) < Mathf.Epsilon)
        {
            result = new Vector3
            {
                x = L1StartPoint.x + L1Direction.x * t1,
                y = L1StartPoint.y + L1Direction.y * t1,
                z = L1StartPoint.z + L1Direction.z * t1
            };
            t = t1;
            return true; // if temp1 = temp2
        }
        return false;
    }
    A11 = L1Direction.y;
    A12 = -L2Direction.y;
    B1 = L2StartPoint.y - L1StartPoint.y;
    det = A11 * A22 - A12 * A21;
    if (Mathf.Abs(det) >= Mathf.Epsilon)
    {
        float d1 = B1 * A22 - A12 * B2;
        float d2 = A11 * B2 - A21 * B1;
        t1 = d1 / det;
        t2 = d2 / det;
        float temp1 = L1StartPoint.x + L1Direction.x * t1;
        float temp2 = L2StartPoint.x + L2Direction.x * t2;
        if (Mathf.Abs(temp1 - temp2) < Mathf.Epsilon)
        {
            result = new Vector3
            {
                x = L1StartPoint.x + L1Direction.x * t1,
                y = L1StartPoint.y + L1Direction.y * t1,
                z = L1StartPoint.z + L1Direction.z * t1
            };
            t = t1;
            return true; // if temp1 = temp2
        }
        return false;
    }
    return false;
}

забыл скриншот :(