you have to set up a funtion that models the volume of the trough..
V = 1/2(b*h)L
the hardest part of these problems is relating the variables to one another. you have to solve for b in terms of h to differentiate, then you can find a derivative that will model the rate at which the volume changes then solve for the rate at which the hieght changes(dh/dt). The easiest way to solve for b in terms of h is to set up a proportion within the triangle. I uploaded a pic at the bottom i hope it helps.