Two circles with centres A and B of radii $3\;cm$ and $4\;cm$ respectively intersect at two points C and D such that AC and BC are tangents to the two circles. Find the length of the common chord

Question

$\begin{array}{1 1} 4.8\;cm \\ 7.2\;cm \\ 2.1\;cm \\ 1.2\;cm \end{array} $