Because you know that the third term is 120, it can be represented as (3,120) and since you know the fifth term is 76.8, you can represent that as (5,76.8). Now that we have this, you can use your slope formula, y2-y1/x2-x1 which finds the rate of change. When you plug in the values, you should have something like this: 76.8-120/5-3
When you simplify, you should get -43.2/2 which would simplify to -21.6, or your rate of change.
