Physics problem, please help!?

Systematic use of solar energy can yield a large saving in the cost of winter space heating for a typical house in the north central United States. If the house has good insulation, you may model it as losing energy by heat steadily at the rate 5700 W on a day in April when the average exterior temperature is 4�C and when the conventional heating system is not used at all. The pive solar energy collector can consist simply of very large windows in a room facing south. Sunlight shining in during the daytime is absorbed by the floor, interior walls, and objects in the room, raising their temperature to 38�C. As the sun goes down, insulating dries or shutters are closed over the windows. During the period between 5:00 P.M. and 7:00 A.M., the temperature of the house will drop and a sufficiently large "thermal m" is required to keep it from dropping too far. The thermal m can be a large quantity of stone (with specific heat 850 J/kg��C) in the floor and the interior walls exposed to sunlight. What m of stone is required if the temperature is not to drop below 18�C overnight?