When the water is x feet deep,

1. draw a picture
2. the volume of water is V(x) = lwh or V(x) = 96x
3. the rate of change of the volume of water is dV/dt
4. the rate of change of the depth is dx/dt
5. a formula relating dV/dt and dx/dt can be obtained by differentiating the final equation in (1) with respect to t. That is, dV/dt = 96 dx/dt
6. Substitute 3 in for dV/dt