A piano turner charges $63 for 1.5 hours of work. Which proportion can be used to find x, the number of dollars the piano turner charges for 4 hours of work?

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.