The cross product of vectors rp and rq will give a vector that is normal to the plane:
... rp × rq = (-3, 6, 9)
Dividing this by -3 (to reduce it and make the x-coefficient positive) gives a normal vector to the plane of (1, -2, -3). Usint point r as a point on the plane, we find the constant in the formula to be zero. Hence, your equation can be written ...
