What is the value of x?

What is the volume of this container?

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.

Idon't get undoing if its pass adding and subtracting so can someone ? x-2 over 5 = 18