Answer
$(a+b+2)(a+b-5)$
Work Step by Step
$(a+b)^{2}-3(a+b)-10$
Let $a+b$ be equal to $x$:
If $a+b=x$, then $(a+b)^{2}=x^{2}$
Rewrite the expression using the new variable $x$:
$x^{2}-3x-10=...$
Factor this expression:
$...=(x+2)(x-5)=...$
Substitute $x$ back to $a+b$:
$...=(a+b+2)(a+b-5)$