Slope: Question 1

A camper lights an oil lantern at noon and lets it burn continuously. Once the lantern is lit, the lantern burns oil at a constant rate each hour. At 2 p.m., the amount of oil left in the lantern is 63 ounces. At 5 p.m., the amount of oil left in the lantern is 61½ ounces. What is the average rate of oil burning, in ounces, every hour?