look at the small triangle of water at the end of the trough ... it is similar to the triangle that forms the end base of the entire trough.

base equals height.

the volume of water in the trough is ...

now you have a water volume formula strictly in terms of h ... take the time derivative, substitute your given info, then solve for what you need.

for the second problem, the edge is increasing at 6 cm/sec, not the volume.

take the time derivative, sub in your given values, calculate

btw, where did "r" come from?