Ravi is planning to buy concert tickets for himself and a small group of friends. the tickets cost $21 each plus a $5 transaction fee. ravi is not sure yet which of his friends are going to the concert. how much can ravi expect to spend on tickets?

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

Graph a should be your answer