Please help with step by step solution

A very long solid piece of rectangular material of 4 m x 3 m has its top face maintained at a constant temperature of 800 K and its left face at 298 K. The bottom face and right face are exposed to an environment at 200 K and the thermal conductivity of the material is 10 W/mK. Use a grid size of Ax= Ay = 0.5 m and calculate the steady-state temperatures of the various nodes