Explanation:
2×length + 2×width = 198 ft
length = width + 9
we use the second equation in the the first :
2×(width + 9) + 2×width = 198
2×width + 18 + 2×width = 198
4×width + 18 = 198
4×width = 180
width = 180/4 = 45 ft
length = width + 9 = 45 + 9 = 54 ft