The three angles of a triangle measure (x+6) , (3x+9) and (4x+5) is the value of x

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.