Width = w=?
Length = l=(3w+8)cm
Perimeter = p = 128cm
Perimeter of a Rectangle = length + length + width + width
Perimeter of a Rectangle =2 length +2 width
P = 2l +2 w
128=2(3w+8)+2w
128=6w+16+2w
128-16=8w
112=8w
112/8=8w/8
14=w
w=14
Width = w= 14cm
Length = l=(3w+8)=3(14)+8 = 42+8 = 50cm