#### Answer

$(7-3x)^2$

#### Work Step by Step

The given expression, $
49-42x+9x^2
,$ is a perfect square trinomial since the first and third terms are both perfect squares and the middle term is twice the product of the square roots of the first and third terms. Hence, the factored form is
\begin{array}{l}
(7-3x)^2
.\end{array}