Answer
$t=4$ hours
Work Step by Step
In 1 hour,
pipe 1 fills $\displaystyle \frac{1}{6}$ of the (full) pool,
pipe 2 fills $\displaystyle \frac{1}{12}$ of the pool.
In $t$ hours, they fill $(\displaystyle \frac{t}{6}+\frac{t}{12})$ of the pool.
We want t for which $\displaystyle \frac{1}{1}$ of the pool is full (the whole job).
$\displaystyle \frac{t}{6}+\frac{t}{12}=1\qquad$
... LCD=$12$
... multiply with $12$
$2t+t=12$
$3t=12$
$t=4$ hours