Answer
-21x$^2$+7
Work Step by Step
The two sets of parentheses contain special products that result in the difference of the squares of the two terms of each set.
(2x+3)(2x-3)-(5x+4)(5x-4)
[(2x)$^2$-(3)$^2$]-[(5x)$^2$-(4)$^2$]
(4x$^2$-9)-(25x$^2$-16)
4x$^2$-9-25x$^2$+16
(4x$^2$-25x$^2$)+(-9+16)
-21x$^2$+7