You can put this solution on YOUR website! LET X BE THE SHORTER LENGTH
LET (200-X) BE THE LONGER LENGTH.
X+Y=200
(200-X)=X+12
200-X=X+12
-X-X=-200+12
-2X=-188
X=-188/-2
X=94 ANSWER FOR THE SHORTER LENGTH.
200-94= 106 ANSWER FOR THE LONGER LENGTH.
PROOF:
106+04=200
200=200