Answer
$(3mn+2)^{2}$
Work Step by Step
Test whether this is a perfect square, $(A+B)^{2}=A^{2}+2AB+B^{2}$
First term: $A^{2}=(3mn)^{2}\Rightarrow A=3mn$
Third term: $B^{2}=(2)^{2}\Rightarrow B=2$
Test:$\qquad$ does $2AB$ equal the middle term?
$ 2AB=2(3mn)(2)=12mn\qquad$ ... yes, it does.
$\Rightarrow$This is a perfect square, $(3mn+2)^{2}$
