Can some explain how to do this?

The units for the area are in square inches or in^2. The approximate value shown above is when using pi = 3.14

Explanation:

Use the pythagorean theorem to find the length of the hypotenuse

a^2 + b^2 = c^2

20^2 + 21^2 = c^2

400 + 441 = c^2

c^2 = 841

c = sqrt(841)

c = 29

The hypotenuse is 29 inches long. This is the diameter of the circle. Half of that is the radius at r = d/2 = 29/2 = 14.5 inches.

The area of the circle is...

A = pi*r^2

A = pi*(14.5)^2

A = pi*210.25

A = 210.25pi

Which is exact in terms of pi

We'll subtract off the triangular region as this isn't shaded in. The area of the triangle is base*height/2 = 20*21/2 = 420/2 = 210 square inches.

So the shaded region is therefore 210.25pi - 210 square inches

This approximates to 210.25*3.14 - 210 = 450.185 when using the approximation pi = 3.14; use more decimal digits of pi to get a more accurate value.