Answer
$9$ hours.
Work Step by Step
Step 1. Assume it takes $x$ hours for the slower plant to fill the tank working alone, then it only takes $\frac{x}{2}$ hours for the faster plant to fill the tank working alone.
Step 2. As working together , it takes $3$ hours, we have $\frac{1}{x}+\frac{1}{(x/2)}=\frac{1}{3}$ which gives $\frac{3}{x}=\frac{1}{3}$ and $x=9$ hours.
