Can someone help me with this math problem?

Consider a parallelepiped, three of whose vertices are given by the position vectors a, b and
c, which do not lie in the same plane.
Determine the vector equation of the plane that passes through the face of the parallelepiped
that contains the point a but does not contain the origin, in the form r ? n = k, where n is a
unit vector and k a constant. Write k in terms of the volume of the parallelepiped and the
area of the face that lies in the plane.

for now I calculated the volume |a*(b*c)|. I also known the area (b*c).

But still I feel like I didn't solved the problem at all. I also calculated (OBxOC).

What should I do next? Pic unrelated.