CP and CQ are tangent 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 $BR= 4\;cm$, then the length of BC is __________

Question

$\begin{array}{1 1} 5\;cm \\ 6\;cm \\7\;cm \\ 8\;cm \end{array}$