The question is as follows:

"A diagonal is a line connecting any 2 points in a polygon. How many diagonals are there in a heptagon?

If the question really means ANY two points then isn't there an infinite number of diagonals? Or if there is a equation to solve this, I certainly haven't learned it. Since this unit is trying to teach applications of permutations and combinations, I'm assuming the question is really only asking about the corner points in which case wouldn't the question be solved by $\displaystyle _{7}C_{2}$?