Models & Qualitative Behavior · Superchallenge problem

A fish's mass $W$ (in grams) follows the von Bertalanffy model $W' = 3W^{2/3} - W$ ($t$ in years): intake grows with surface area, upkeep with mass. It starts at $W(0) = 1$. At what time is its growth rate $W'$ largest? Give an exact answer.