The key word here is *square* in "square flower garden".
So we need to find the area of the flower garden, and subtract it from the overall area of the yard. The remainder will be how much grass covers the yard.
Area of the Square flower garden:
7.5 * 7.5 = 56.25
Area of the Entire yard:
25 * 18 = 450
450 - 56.25 = 393.75 square feet of grass
