Chapter R - Section R.4 - Factoring Polynomials - R.4 Exercises - Page 40: 103

$4xy$

Using $a^2-b^2=(a+b)(a-b)$ or the factoring of the difference of two squares, then the factored form of the expression, $ (x+y)^2-(x-y)^2 ,$ is \begin{array}{l}\require{cancel} [(x+y)-(x-y)][(x+y)+(x-y)] \\\\= (x+y-x+y)(x+y+x-y) \\\\= (2y)(2x) \\\\= 2(y)2(x) \\\\= 4xy .\end{array}