Statistics II- multiple regression analysis

Cellulon, a manufacturer of home insulation, wants to develop guidelines for builders and consumers on how the thickness of the insulation in the attic of a home and the outdoor temperature affect natural gas consumption. In the laboratory, it varied the insulation thickness and temperature. A few of the findings are:
 
Monthly Gas Consump. (cubic feet),Y   Thickness of Insulation(inches), X1   Outdoor Temp(ºF),X2
30.3                                                                          6                                        40   
26.9                                                                          12                                             40   
22.1                                                                           8                                        49   
 
On the basis of the sample results, the regression equation is:
 
Y = 62.65 – 1.86X1 – 0.52X2
 
a. How much natural gas can homeowners expect to use per month if they install 6 inches of insulation and the outdoor temperature is 40 degrees F? (Round your answer to 2 decimal places.)
cubic feet:_____?
 
b. What effect would installing 7 inches of insulation instead of 6 have on the monthly natural gas consumption (assuming the outdoor temperature remains at 40 degrees F)? (Round your answers to 2 decimal places.)
A decrease of  ____ cubic feet of monthly natural gas consumption, down to _____ cubic feet.