Notice, "x" is a variable, so it varies, it will take on many values.
usually car dealers will say, you need to pay some processing fee, a flat-fee, and after that, you get charged for the "wear and tear" of the car, so by the mileage you drive, the more miles you drive, the more wear and tear, therefore, the more you pay, the less you drive, the less you pay.
0.15x <---- 15 cents for every "x", miles
if you drive 200 miles you owe 0.15(200)
300 miles the 0.15(300) and so on.
the 22, is never changing in that equation, is a constant, just like the flat-fee.