I need help please i can’t figure it out

If f(x)=3x+1 and f^-1=x-1/3, then f^-1(7)=

Can someone tell me if this is perpendicular? !

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.