The centroid of a triangle is 1/3 the distance from the midpoint of the side to the opposite vertex. (Put this in your notebook of geometric relationships for triangles.)
.. QP = (1/3)QN
.. PN = (2/3)QN
4. PN = 2/3*21 = 14
.. QP = 1/3*21 = 7
6. PN = 2/3*42 = 28
.. QP = 1/3*42 = 14