If the garden is on a grid, you can either count out the sides or math it. As I don't have a grid in front of me, I'll math it.
the lowest horizontal (y-axis) point is 6, and the highest is 9. subtract, and the vertical length of the garden is 3. The lowest vertical (x-axis) point is 3, and the highest is 8. subtract, and the length horizontally is 5.
Since we need the area, we multiply the sides. 3 x 5 is 15.
The area of the garden is 15 square feet.