c. length = 9 feet; width = 6 feet
Work Step by Step
Let $l$ be the length of the rectangle. Let $w$ be the width of the rectangle. Length is 3 feet longer than the width. $l=w+3$ Perimeter is 30 feet. $2l+2w=30$ $l=w+3$ $2l+2w=30$ $2(w+3)+2w=30$ $2w+6+2w=30$ $4w+6=30$ $4w=24$ $w=6$ $l=w+3$ $l=(6)+3$ $l=9$ Length is 9 feet and width is 6 feet, so the answer is c.