Let x be the length, y be the width and p be the perimeter.
p = 2x + 2y
Since the length is 3 times the width
x = 3y
So,
2400 = 2(3y) + 2y
2400 = 6y + 2y
2400 = 8y
y = 300
Now plug y back in,
x = 3(300)
x = 900
The length is 900 ft and the width is 300 ft