#### Answer

$144$

#### 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 $m=-24$
\begin{align*}
&=\left(\frac{-24}{2}\right)^2\\\\
&=\left(-12\right)^2\\\\
&=144
\end{align*}