River Y
c=33
This has a flat cost no matter what the hours
River Z
c = 5n + 13 where n is the hours and 13 is the deposit
Solving the equations
We set the two equations equal to each other when the cost is the same
33 = 5n+13
Subtract 13 from each side
33-13 = 5n
20 = 5n
Divide each side by 5
20/5 = 5n/5
4 = n
They would be the same cost at 4 hours rental.