help me

半徑15cm 圓周 2pi(15) = 30pi cm
(25/2) pi 即 (25/2) / 30 = 5 / 12 個圓
c = 360 (5/12) = 150 度

半徑3cm 圓面積 pi(3)^2 = 9pi cm^2
扇形的面積是 (21/4)pi cm^2 ??
如是的話, 即 (21/4) / 9 = 7 / 12 個圓
y = 360 (7/12) = 210 度

圓心角270 扇形, 即 270/360 = 3/4 個圓
圓面積為 27pi / (3/4) = 36pi cm^2
半徑 z = 開方36 = 6 cm