The total of all 3 sides in the first one is the perimeter, and since you have 2 of the side measures, subtract both of those from the perimeter of 6x to find the missing side. The missing side is 3x-7. In the second one, since the opposite sides of rectangles are congruent 3x-3 is the measure for two sides. Subtract 6x-6 from 10x+6 for those sides, and you should be left with 4x + 12. Divide that by two since there are two side lengths left with the same value, and you should get 2x + 6 as the missing side.