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A rectangle has four sides, consisting of two pairs of opposite sides where one pair is a certain length and the other pair is a different length. The sum of all four sides is the perimeter.

You have been told the length of one side, so you know the length of the opposite side, correct? Then the perimeter minus these two sides is equal to the combined length of the other two sides - what do you get for that? Finally, since that other pair is equal to each other, divide their combined length by two to find the length of each of those sides.

It helps to draw it out as a picture to double check your work. Post back with what you get for an answer and we'll check it.