Explanation:
it is the area of the square minus 4 quarter circles (which means 1 full circle) with radius of 10 ft (the half of a side of the square).
we know how to calculate the area of a square "As"
As = side²
we know how to calculate the area of a circle "Ac"
Ac = pi×radius²
so, the shaded area "Ashade"
Ashade = side² - pi×radius² = 20² - pi×10² =
= 400 - pi×100 = 85.84073464... ft²
I don't know, if you need to round the answer in some way.
if it is to e.g. 2 digits after the decimal point (hundredths), then it would be
85.84 ft²