we know that
When circles have the same central angle measure, the ratio of measure of the sectors is the same as the ratio of the radii squared.
so
rA²/rB²=A/B
where
rA-------> radius circle Q
rB------> radius circle R
A------> area sector Q
B------> area sector R
rB²=B*rA²/A-------> 7²*[9π]/[4π]-------> 110.25--------> rB=√110.25
rB=10.5 cm
the answer is
radius of circle R is 10.5 cm