What is the average rate of change for the function y= 2(3)^x over the interval x=0 to x=3 Thank

In order for the function to be linear, what must m be and why? m = 22 because the rate of change is 1. m = 20 because the rate of change is 3. m = 22 because the rate of change is -1. m = 20 because the rate of change is -3.