Explanation:
the perimeter of a rectangle is
2×length + 2×width
in our case this is 48 cm.
length = width + 3
now we use this identity in the original equation :
2×(width + 3) + 2×width = 48
2×width + 6 + 2×width = 48
4×width = 42
width = 42/4 = 10.5 cm
length = width + 3 = 10.5 + 3 = 13.5 cm