Answer
$t=4.5$ hours
Work Step by Step
In 1 hour,
the first pipe fills $\displaystyle \frac{1}{3}$ of the pool's volume,
and the second drains $\displaystyle \frac{1}{9} $ of the pool's volume.
In $t$ hours, $\displaystyle \quad\frac{t}{3}-\frac{t}{9} \quad$ of the pool is filled
We want the t (in hours) when this equals $\displaystyle \frac{1}{1}$ of pool volume.
$\displaystyle \frac{t}{3}-\frac{t}{9}=1\qquad.../\times 9$
$3t-t=9$
$2t=9$
$t=4.5$ hours
