Was your other forum post asking about 7.21 from Counting and Probability as well? It does involve Geometry.
For 7.21, you want to try to find how many diagonals an equilateral triangle, a square, a regular pentagon, and a regular hexagon have. For example, the regular octagon (pictured below) has $5+5+4+3+2+1 = 20$ diagonals (the dotted lines).
As a hint for 7.22, it also involved geometry! Think about other things you know about triangles, quadrilaterals, pentagons, hexagons, etc.
Hope these hints help!