3) Let the length of missing side be y then,
(2x-5) + (3x+9) +y = 7x+12
or 5x +4 + y = 7x +12
or y = 7x -5x +12 -4
or y = 2x + 8
Length of missing side = 2x+8
(4) Let the breadth of the rectangle be y then,
2((3x+2) + y) = 14x-8
or 6x + 4 +2y =14x-8
or 2y = 8x -12
or y = 4x - 6