To find perimeter, we use this equation.
2L+2W=42
Let's say the longer side is the length.
L=W+7
Let's plug that in for L.
2(w+7)+2w=42
2w+14+2w=42
Add like terms.
4w+14=42
Subtract 14 from both sides.
4w=28
Divide.
28÷4=7=W
The shorter side is 7, which means the longer side is 14 cm.