Minimizing area of a poster with fixed printed area

I have a calc problem I am having problems with.

The top and botom margins of a poster are 6cm and the side margins are each 4cm. If the area of printed material on the poster is fixed at 384cm^2 find the dimensions of the poster with the smallest area.

I am just having problems setting up an equation to start solving this problem. I tried setting up a formula for finding the total area of the entire poster then subtracting the area of the printed area but that didnt seem to work. Can someone help me out? Thanks