Answer
$(3n-7)^{2}$
Work Step by Step
$9n-42n+49=$
...write first and last terms as squares.
$=(3n)^{2}-42n+7^{2}$
...does middle term equal $2ab$?
$42n=2\cdot(3n)\cdot(7)$
$=(3n)^{2}-2\cdot(3n)\cdot(7)+7^{2}$
...write as the square of a binomial.
$=(3n-7)^{2}$
