Maybe you need a formula that you can use; start with

2L + 2W = P

Divide both sides by 2: (2L + 2W) / 2 = P / 2

L + W = (P / 2)

Subtract W from both sides: L + W - W = (P / 2) - W

L = (P / 2) - W

That's a generic formula; substitute what you already know, i.e., P = 500 ft

L = (500 ft / 2) - W = 250 ft - W

Now, choose any Width less than 250 ft, plug it into the formula, and it will give you the length.

W = 50 ft: L = 250 ft - 50 ft = 200 ft 500 ft = (2 * 200 ft) + (2 * 50 ft)

W = 85 ft: L = 250 ft - 85 ft = 165 ft 500 ft = (2 * 165 ft) + (2 * 85 ft)

Something is missing from the problem statement, preventing us from reaching the needed answer.