Each piece of cardboard is 0.3 centimeters thick. If Tyler stacks 5 pieces of cardboard on top of one another, how thick will the stack be?

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.