Derive the mathematical 3-D heat conduction equation in Spherical Coordinates?

A furnace is made up of three layers of thicknesses 250 mm, 150 mm nad 100 mm with thermal conductivities 1.65, k, 9.2 W/m ºC respectively. The inside is exposed to gases at 1250 ºC with a convection coefficient of 25W/m2 ºC and the inside surface is at 1100 ºC, the outside surface is exposed to air at 25 ºC with convection coefficient of 12W/m2 ºC. calculate:

i. The unknown thermal conductivity (k);

ii. The overall heat transfer coefficient;

iii. All surface temperatures.