Simple..
you have y which is the cost of the tickets..and you have x which is the number of tickets..
So, you have those points on the graph....find an equation!
Take any of those coordinates..and find the equation of the line...in the form of y=mx+b
(2,10) and (5,25)
Using:


Thus, you know your slope. m=5
But, remember it's in the form of y=mx+b
So, use any of the coordinates...let's use (2,10)
10=5(2)+b
10=10+b
0=b
Thus, your equation is... y=5x
Thus, your answer.