Part A:
We multiply 1000 square feet by 60,000 BTUs/sq ft. and get 60000000 BTUs to heat the entire house. So, 6*10^7 is our answer for Part A. (6 followed by 7 zeroes.
Part B:
We can divide 6*10^7 by 10^3 and get 6*10^4. So, 6*10^4 is our answer for Part B. (60000 cubic feet of gas are required to heat the house.