Answer
$(a-3b-3)^{2}$
Work Step by Step
Test whether this is a perfect square, $(A-B)^{2}=A^{2}-2AB+B^{2}$
First term: $A^{2}=(a-3b)^{2}\Rightarrow A=a-3b$
Third term: $B^{2}=(3)^{2}\Rightarrow B=3$
Test:$\qquad$ does $-2AB$ equal the middle term?
$-2AB=-2(a-3b)(3)=-6(a-3b)\qquad$ ... yes, it does.
$\Rightarrow$This is a perfect square,
$[(a-3b)-3]^{2}=(a-3b-3)^{2}$
