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:

$\displaystyle V(t) = 10^9 + 10^8(1-e^{-t/100}) - rt$

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 $\displaystyle x$ percent of its initial value of $\displaystyle 10^9$ L. The inputs of the second function should be $\displaystyle x$ and $\displaystyle r$.

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.