In this case the perimeter is 48 and one side is 12 cm longer so let’s make the unknown side W, and L as 12+w since the longer side is 12cm longer then the unknown side.
L=W+12 2L+2W=48 2(W+12)+2W=48 2W+24+2W=48 4W=48-24 4W=24 W=24/4 W=6 ANS. FOR THE WIDTH. L=6+12=18 ANS. FOR THE LENGTH. PROOF: 2*18+2*6=48 36+12=48 48=48