I'll make up a similar example and show how to solve that.
My uber rides cost a base minimum fee of $5. You get charged another $2.25 for every mile I drive you on your trip.
The minimum fee represents a constant, and the mileage is a variable. The price also varies.
So, the equation can be written price = base minimum + miles(2.25)
let p = price
m = miles
p = 5 + 2.25m
how much is a 10 mile uber ride with me?
well what variable is miles? m,
plug in 10 for m and were good.
p = 5 + 10(2.25)
= 5 + 22.50 = 27.50
im an expensive uber driver