Answer :

in this situation you have to remember that this is going to be a rectangle so the with is going to be 3 and the opposite side will be the same size.
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if one side is 3 the opposite side is also 3 if you subtract their sum from the perimeter 16 it leaves 10 divided by 2 the other sides are 5. Area =bXh. 3x5=15 which is the area