We will use "y" to represent the amount of money left and the function y = mx + b.
y = 2x + 30
This is your answer!
The y is how much money he has left after x amount of rides, the 2 is the $2.00 he spends per ride, the x is how many rides he goes on, and the 30 is how much he has already spent. I he rides 5 ride he would have spent $40.
Hope this helps!