A closed box is filled with dry ice at a temperature of -78.5 C, while the outside

Question:

A closed box is filled with dry ice at a temperature of -78.5 °C, while the outside temperature is 21.0 °C. The box is cubical, measuring 0.350 m on a side, and the thickness of the walls is 3.00 × 10-2 m. In one day, 3.10 × 106 J of heat is conducted through the six walls. Find the thermal conductivity of the material from which the box is made.