Answer
$x^2+2xy+y^2-9$
Work Step by Step
The given expression can be written as:
$[(x+y)+3][(x+y)-3].$
The expression above is of the form $(a+b)(a-b)$.
RECALL:
(1) $(a+b)(a-b) = a^2-b^2$
(2) $(a+b)^2=a^2+2ab+b^2$
Multiply the expression using formula (1) above with $a=x+y$ and $b=3$ to obtain:
$=(x+y)^2 - 3^2
\\=(x+y)^2-9$
Square the binomial using formula (2) with $a=x$ and $b=y$ above to obtain:
$=x^2+2xy+y^2-9$
