Here's a pic of the situation :

The intersection point of the side of the rectangle and the circle at F = (sqrt(25^2 -7.5^2), 7.5)
And, by symmetry.....the central angle of the sector is given by 2* tan-1(7.5/ sqrt(25^2 -7.5^2)) = about 34.915206247444°
So.....the area of the rectangle outside the sector = 375 - pi*(25)^2* [ 34.915206247444 / 360) = [375 - 190.432] cm^2 ≈ 185 cm^2
