For a fish swimming at a speed relative to the water, the energy expenditure per unit time

Question:

For a fish swimming at a speed relative to the water, the energy expenditure per unit time is proportional to v3. It is believed that migrating fish try to minimize the total energy required to swim a fixed distance. If the fish are swimming against a current u (u (a) Determine the value of that minimizes E.
(b) Sketch the graph of E.
Note: This result has been verified experimentally; migrating fish swim against a current at a speed 50% greater than the current speed.