Answer
$a.\quad x(x+y)-y(x+y)$
$ b.\quad (x+y)(x-y)$
Work Step by Step
$a.$
From the rectangle with side lengths $x$ and $x+y$ subtract the rectangle with side lengths $y$ and $x+y$ .
$x(x+y)-y(x+y)$
$b.$
$x(x+y)-y(x+y)$= ... factor out $(x+y)$
$=(x+y)(x-y)$
