## Elementary Algebra

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: $(-3+7i)(-3-7i)$ =$(-3)^{2}-(7i)^{2}$ =$9-49i^{2}$ =$9-49(-1)$ =$9+49$ =$58$