Let the width be x.
The perimeter is 16x.
The length is x + 12.
The perimeter is twice the length added to twice the breadth, so we have:
16x = 2(x + 12) + 2x
16x = 2x + 24 + 2x
16x = 4x + 24
16x - 4x = 24
12x = 24
x = 24 / 12
x = 2 cm
x + 12 = 12 + 2 = 14 cm
Hence, the length is 14 cm and the width is 2 cm.