Try this solution (note, this is not the shortest way):
1. if the given length is 'l' and the width is 'w', then according to the condition 4*l=w;
2. according to the condition the perimeter is 30 ft. Formula of the perimeter is P=2(l+w), then 2(l+w)=30;
3. if to substitute '4*l' instead of 'w', the formula of the perimeter (from item no. 2) will be as 2(4*l+l)=30 or 10*l=30 ⇒ l=3 ft. The length is 3 ft.!
4. if l=3 ft., then w=4*3=12 ft. The width is 12 ft.!
5. Area is
A=w*l; A=12*3=36 ft²
