==> ${n_1} = \frac{v}{{2l}}$ and ${n_2} = \frac{v}{{2l + 2\Delta l}}$
==> No. of beats $ = {n_1} - {n_2} = \frac{v}{2}\left( {\frac{1}{l} - \frac{1}{{l + \Delta l}}} \right) = \frac{{v\Delta l}}{{2{l^2}}}$

The amplitude of the particle at $x =\frac{4}{3} \,cm$ will be........ $cm$.