Q: What are the properties of the plane ?

If O is the origin and a point C has position vector c, find:

a) the length of the projection of OC onto the plane

b) the distance of C from the plane.

For part a) this sound like it has something to do with the scalar product?

For part b) I know how to find the distance of a point to a plane using the formula:

Help would be hugely appreciated.