Basically, the problem is asking how many diagonals does a (i) equilateral triangle, (ii) square, (iii) regular pentagon, and (iv) regular hexagon have?

Remember that a diagonal connects two vertices of a shape (but is not a side of the shape). As a simple example, a square has 2 diagonals connecting "opposite" sides. Hope this helps!

Note: It isn't important for answering 7.21, but there is a fairly nice pattern that this sequence would have in general. That is, if you let $a_n = $ the number of diagonals in a shape with $n$-sides, there are some nice patterns and formulas that show up.