Thank you for posting your question here at brainly. Below are the solution:
Let (a,b) be the coordinates of the mass center.
By symmetry, b = 0.
To get a, consider x^2+y^2 = R^2 in the first quadrant,
a
= (4/ piR^2) ∫x sqrt(R^2 - x^2)dx, from 0 to R
= (4/ piR^2)(1/2)(2/3)(R^2)^(3/2)
= 4R/(3pi)
I hope the answer will help