#### Answer

$(3n-7)^{2}$

#### Work Step by Step

Given the polynomial
$(3n)^{2}$ - 42n + $(7)^{2}$
We see that the polynomial has the first and last term squared and the middle term is -2 times the first and last term. Thus it follows the rule of
$a^{2}$-2ab+$b^{2}$=$(a-b)^{2}$
In this polynomial a= 3n and b=7
$(3n)^{2}$ - 2(3n)(7) + $(7)^{2}$ = $(3n-7)^{2}$