A) What area of a black body kept at 300°C radiates 100W?

B) If a 25m2 roof has a U-value of 0.4Wm^-2 °C^-1, how much heat, expressed in MJ, would escape through it over a day if the temperature inside the building was maintained at 7°C above the outside temperature? can anyone please just tell me what Fourmile shall I use?