how did they get D2, D3, and D4?

A steel tube (k=47Wm1K1) has an internal diameter of 8cm and thickness 5.5mm. The tube is 4m long and is covered with a first layer of insulation (k=0.5Wm1K1) of thickness 9cm. A second layer of insulation (k=0.25Wm1K1)4cm thick covers the first layer. Knowing that the outside temperature of the second layer is 293K and the thermal flux from the outside to the environment is 400Wm, calculate: (i) The thermal power (rate of heat transier) for the entire tube (ii) The temperature at the interface between the second and the first insulation layer and the temperatures on the outside and inside surfaces of the tube