The area of a rectangle is the product of length and width.
A = length*width
Jarel's garden has an area of
A = (9 ft)*(3 ft) = 27 ft²
Morgan's garden has an area of
A = (6 ft)*(6 ft) = 36 ft²
The difference in area of the two gardens is
36 ft² - 27 ft² = 9 ft²
The appropriate choice is
B. 9 square feet