Answer
$81$
Work Step by Step
RECALL:
To complete the square for $x^2 +mb$, just add $\left(\dfrac{m}{2}\right)^2$.
Using the rule above, the term that must be added to the given expression to complete the square is:
With $m=18,$
\begin{align*}
&=\left(\frac{18}{2}\right)^2\\\\
&=9^2\\\\
&=81
\end{align*}