I need help on understanding what the question is asking.
A fish swims at a speed v relative to the water, against a current of 9 mi/h. Using a mathematical model of energy expenditure, it can be shown that the total energy E required to swim a distance of 18 mi is given by
E(v) = 2.73v^3(18/v − 9)
.
Biologists believe that migrating fish try to minimize the total energy required to swim a fixed distance.

Find the value of v that minimizes energy required.

Question

I need help on understanding what the question is asking.
A fish swims at a speed v relative to the water, against a current of 9 mi/h. Using a mathematical model of energy expenditure, it can be shown that the total energy E required to swim a distance of 18 mi is given by
E(v) = 2.73v^3(18/v − 9)
.
Biologists believe that migrating fish try to minimize the total energy required to swim a fixed distance.

Find the value of v that minimizes energy required.