AFAIK the volume of a trapazoid like this should be

[tex]V = \frac{1}{2}h(a + b)l[/tex]

where a, b, and l are all given constants, therefore

[tex]\frac{dV}{dt} = \frac{1}{2}\frac{dh}{dt}(a + b)l[/tex]

But this can't be right. I don't think I'm approaching this correctly...