In Fig. CP and CQ are tangents from an external point C to a circle with centre O. AB is another tangent which touches the circle at R. If CP = 11 cm and BR = 4 cm, find the length of BC.

**Hint:** We have, CP = 11 cm

\(\therefore\) CP = CQ = CQ = 11 cm

Now, BR = BQ [Tangents drawn from B)

\(\Rightarrow BQ=4\) cm

\(\therefore\) BC = CQ - BQ = (11 - 4)cm = 7 cm