The help page for Plot says that
Plot[{somethingWithXInIt,somethingElseWithXInIt},{x,someLowerLimit,someUpperLimit}]
(when you plug in appropriate things for each of those some... will plot two things on top of each other on the same graph.
If you think a little can you see how that might be useful for the problem you are working on? Did they perhaps mention anything like this in class?
Hopefully that hint will be enough to get you started.