please solve part b and part c

14.1 (a) A cube of 'hot rock' of side h has its top surface at a depth d below the Earth's surface. The rock has a density p and spe- cific heat capacity c. The material above the cube has thermal conductivity k. If the rock is treated as an isothermal mass at temperature T above the Earth's surface with no internal heat source, show that the time constant for cooling is given by: phcd k (b) Calculate t for a cubic mass of granite (of side 10 km, density 2.7 x 103 kg/m3, specific heat capacity 0.82 x 103 J kg-K-), positioned 10 km below ground under a uniform layer material of thermal conductivity 0.40 J m-'s-K-1 T= (c) Compare the natural conductive loss of heat from the granite with commercial extraction at 100 MW from the whole mass