Let's first convert this to numbers. When a number is decreased by a certain amount, that is the same as saying that something is subtracted from it. Therefore:
If a number (Let's call it x) has been subtracted by 7 (x - 7) then to find the previous number you just do the opposite. Let's call the end result y. 2 + 7 = x (Or y + 7 = x). 2 + 7 = 9.
