P=2L+2W where L is the length and W is the width. If the length is 50 feet longer than the width, we can write the expression L=50+W. Substituting into P=2L+2W we get P=2(50+W)+2W=100+2W+2W=100+4W. Since P=300, we can say 300=100+4W. Subtracting 100 from each side we get 200=4W. Dividing each side by 4 gives 50=W. Substituting this into L=50+W gives L=50+35=85. The length is 85 feet.
