To find the number of fish \( f \) in the bucket before Lester feeds the dolphins, we know that he divides the total number of fish equally among 3 dolphins, and each dolphin receives 6 fish.
We can express this scenario with the equation:
\[ f = 3 \times 6 \]
This simplifies to:
\[ f = 18 \]
So, the equation you can use to find the number of fish \( f \) in the bucket is:
\[ f = 3 \times 6 \]