Answer
b., $x^{2}-49$
Work Step by Step
$(x+7)(x-7)$
As shown on page 286, the product of the sum and difference of a two terms is given by the following formula:
$(a+b)(a-b)=a^{2}-b^{2}$
In this case, $a=x$ and $b=7$.
$(x+7)(x-7)=x^{2}-7^{2}=x^{2}-49$
