#### Answer

$-\text{ }\text{ }7$

#### Work Step by Step

The given expression can be written as:
$=x^2-2(x)(7)+7^2$
RECALL:
The perfect square trinomial $a^2-2ab+b^2$ can be factored as $(a-b)^2$.
Factor the trinomial using the formula above with $a=x$ and $b=7$ to obtain:
$=(x-7)^2$
Thus, the missing expression in the given statement is:
$-\text{ }\text{ }7$