Income Statement expense equals Accounts payable and notes payable. When is this possible?

My professor gave me a problem that if we solve we don't have to take the final. On a recent test we took we had to calculate the expense on an income statement. Most did it on the standard principal revenue - expense = Net earnings/loss however some did it by adding up accounts payable and notes payable. My professor said that it works only in certain situations. What is this situation!!!