I'm just reading through my algebra textbook and one of the example questions is solve the following

(2x+3)(2x+5)-4(x+9)(x-5)

where the answer is supposed to be an integer. I don't see how it can be an integer though because the xs don't seem to cancel out. Maybe I've done something wrong but this is what I've got so far:

2x^{2}+4x+21-4(x^{2}+4x-45)

=2x^{2}+4x+21-4x^{2}+8x-180

= -2x^{2}+21-4x-180

That's as far as I've managed to get. I probably messed up somewhere because I'm pretty new to these sorts of exercises but I can't see how the xs would cancel each other out when they're not the same value. Or maybe they are and I'm just missing something really obvious. Would greatly appreciate any help clarifying this. Thanks.