[Calculus 1] Optimization

I am having troubles starting this problem. A farmer wishes to use 240 meters of fencing to surround 2 identical adjacent rectangular enclosures. What should be the dimensions of each enclosure if the total area is to be a maximum? I know in order to to this type of problem you need 2 equations and then I think take the derivative of one to find the max? But I don't know the equations or how to set it up.