Start by defining your variables. Let the length be L and the width W. Note that W = 2L/3.
Use the Perimeter formula: P = 2W + 2L. Substituting,
P = 2[2L/3] + 2L, or P = 4L/3 + 2L, or P = 4L/3 + 6L/3 = 10L/3 = 210 feet
Then 10L = 630 feet, and L = 63 feet. Thus, W = 2L/3 = 2(63 feet)/3 = 42 feet.