Related Rates Problems (two)

Alright, I will need some assistance for two problems that I face. Thanks in advance.

A trough is 12 feet long and 3 feet across the top (see figure). Its ends are isosceles triangles with altitudes of 3 feet. http://www.webassign.net/larson/2_06-26.gif (a) If water is being pumped into the trough at 2 cubic feet per minute, how fast is the water level rising when

*h* is

1.9 foot deep?

(b) If the water is rising at a rate of

http://www.webassign.net/www28/symIm...2b82d56e22.gif inch per minute when

*h* =

2.5, determine the rate at which water is being pumped into the trough.

Honestly I'm not sure how to do this one at all, I need an explanation.

AND

All edges of a cube are expanding at a rate of

6 centimeters per second. (a) How fast is the volume changing when each edge is

3 centimeter(s)?

-I set dV/dt as 6 cm/s

-We must find dx/dt when x=3cm

The eq. is V=x^3

So dV/dt = 3x^2 (dr/dt)

so I solved for dr/dt, dr/dt = dV/dt*1/3x^2. Then I plugged in 3 for x, but this wasn't correct, where did I go wrong? Thanks a lot guys, much appreciated.