Need Help with Function files and Fzero command (Homework problem MATLAB)

The problem reads like so:

Using estimates of rainfall, evaporation, etc. The town engineer developed the following model for the amount of water in the town reservoir as a function of time:

where V is water volume in liters, t is time in days, and r is the towns consumption rate in liters per day. Write two user defined functions. The first function should define V(t) for use with the *fzero* function. The second function should use *fzero* to compute how long it will take for the water to decrease to percent of its initial value of L. The inputs of the second function should be and .

to be totally honest I don't have the foggiest of an idea on where to start. I'm not looking for anyone to hand me the answer, I just want to be pointed in the right direction.