A trough is 15ft long adn 4ft across the top. Its ends are isosceles trianngles with height 3ft. Water runs into the trough at the rate of 2.5

/min. How fast is the water level rising when it is 2 ft deep?

[The figure is an upside down triangular prism with the information above & water being "poured in" from a cup]

Okay...I tried to put down all the information I have:

the amount of water, lets say "l" -- > l=2ft

dl/dt is what we're looking for.

dV/dt = 2.5

Now I'm supposed to use an equation to figure this out. That's where I'm stuck.