Really you just need to plug in each one. So 4 plugged into x in the equation comes out to be 87. And if you plug in 7 it comes out to be 93. so if you take 93 subtract 87 from it, it comes to be 6. and if you take 7 and subtract 4 from it you get 3. So when we plug it in it doubles. So the average rate of change(slope) is 2. because 6/3=2. Average rate of change is the exact same thing as slope.
