The length of the arc subtended by the sector of angle 120° = circumference of the base of the cone.

\(\frac{120}{360} \times 2 \times \frac{22}{7} \times 21 = 2\pi r\)

\(44 = 2\pi r\)

\(r = 22 \div \pi\)

\(r = 22 \times \frac{7}{22}\)

r = 7 cm