Answer
$$ \frac{36}{7}{hours}$$
Work Step by Step
We add the rates to find their combined rate:
$$ \frac{1}{9}+\frac{1}{12}\\ \frac{4}{36}+\frac{3}{36}\\ \frac{7}{36}$$
Thus, taking the reciprocal, their combined time is:
$$ \frac{36}{7}{hours}$$
