25x^2 - 64y^2
The first thing you can do is look for a subtraction sign. The word difference implies subtraction, so you can rule out any expressions that add terms. Secondly, you can look for squares. 17x^2 - 23y^2 is a subtraction expression, but 17 and 23 are not squares. However, 25x^2 - 64x^2 is a subtraction expression and its terms are squares. 25 = 5x5, 64 = 8x8. Hope this helps!
