Hey mcleja.

Hint: The distance between the point and the plane is given by n . p + d where n is the normal and p is the point with plane equation ax + by + cz + d = 0 where the normal is (a,b,c).

You then move the point along the normal by calculating p' = p - n*(n.p + d) which "backtracks the point" along the normal in the reverse distance.