A Carnot heat engine receives heat from a TER at TTER through a heat exchanger where the

Question:

A Carnot heat engine receives heat from a TER at TTER through a heat exchanger where the heat transfer rate is proportional to the temperature difference as QH = A(TTER-TH). It rejects heat to a cold reservoir at TC. If the heat engine is to maximize the work output, show that the high temperature in the cycle should be selected as TH = sqrt(TTERTC).