## Elementary Algebra

First, we use the formula: $(a+b)(a-b)=a^{2}-b^{2}$. Then, knowing that $i=\sqrt{-1}$, we can substitute $-1$ for $i^{2}$ to obtain: $(2+9i)(2-9i)$ =$(2)^{2}-(9i)^{2}$ =$4-81i^{2}$ =$4-81(-1)$ =$4+81$ =$85$