Answer
$100$
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 $m=20$
\begin{align*}
&=\left(\frac{20}{2}\right)^2\\\\
&=\left(10\right)^2\\\\
&=100
\end{align*}