#### Answer

$(a-b+3)(a+b-3)$

#### Work Step by Step

RECALL:
A difference of two squares can be factored using the formula:
$x^2-y^2=(x-y)(x+y)$
Factor the given difference of two squares using the formula above with $x=a$ and $y=b-3$ to obtain:
$=[a-(b-3)][a+(b-3)]
\\=(a-b+3)(a+b-3)$