Explanation:
the perimeter of a rectangle is
2×length + 2×width
in our case
length = width + 3
and
2×length + 2×width = 30
using the first equation in the second :
2×(width + 3) + 2×width = 30
width + 3 + width = 15
2×width + 3 = 15
2×width = 12
width = 12/2 = 6 ft
length = width + 3 = 6 + 3 = 9 ft