a rectangle has 4 sides, each set of opposite sides are congruent, perimeter is the distance around an object, so we need to add up all 4 sides and it should equal 280cm, we know one side is 76, so our equation should look as follows:
76 + x + 76 + x = 280
2x + 152 = 280
2x = 128
x=64
you can check the answer but substituting 64 for x and seeing if it comes out to 280
