The independent variable is the number of concert goers which we can call x. The function f(x) represents the total cost of the tickets. We can write:
f(x) = $21 * x + $5, where the * symbol represents multiplication
So the total costs are:
x=1: f(1) = $26
x=2: f(2) = $47
x=3: f(3) = $68
