It gives you two data points, (5, 350) and (6.5, 455). Constant rate of change also means slope, and the formula for slope is (y(1)-y(2))/(x(1)-x(2)). If you plug in the data points you got for this, you get (350-455)/(6.5-5), which is equal to 105/1.5. Drop that in a calculator and you get 70.
