Consider a Carnot cycle heat engine operating in outer space. Heat can be rejected from this engine

Question:

Consider a Carnot cycle heat engine operating in outer space. Heat can be rejected from this engine only by thermal radiation, which is proportional to the radiator area and the fourth power of absolute temperature, Qrad ~ KAT4. Show that for a given engine work output and given TH, the radiator area will be minimum when the ratio TL/TH = 3/4.