jmc

Let $w$ be the width of the original rectangular garden. The perimeter of the rectangle is $2(w+2w)=6w$, so the perimeter of the square is $12w$. The dimensions of the square are $3w\times 3w$ and its area is $(3w)(3w)=9w^2$, so set $9w^2=3600{\text{ ft.}}^2$ to find $w^2=400$ square feet. The area of the original rectangle is $(2w)(w)=2w^2=2\cdot 400{\text{ ft.}}^2=\boxed{800}$ square feet.