Answer
58
Work Step by Step
We use the formula $(a-b)(a+b)=a^{2}-b^{2}$ with $a=-3$ and $b=7i$. Then, knowing that $i=\sqrt{-1}$, we can substitute $-1$ for $i^{2}$ to obtain:
$(-3+7i)(-3-7i)$
=$(-3)^{2}-(7i)^{2}$
=$9-49i^{2}$
=$9-49(-1)$
=$9+49$
=$58$
