The arc measurement of the sector is 216°.
Therefore the area of the sector is 216/360 = 0.6 of the area of the entire circle.
Let r = the radius of the circle.
Then the area of the circle is πr².
Therefore the area of the sector is
0.6*πr²
Because the area of the sector is 15π cm², therefore
0.6πr² = 15π
r² = 15/0.6 = 25
r = 5 cm
Answer: 5 cm