Answer
$$v=4.85 \mathrm{m} / \mathrm{s}$$
Work Step by Step
As string extend its lowest point, its original potential energy $U=m g L$
(measured respective to the lowest point) is changed into kinetic energy. Thus,
$$m g L=\frac{1}{2} m v^{2} \Rightarrow v=\sqrt{2 g L}$$
With $L=1.20 \mathrm{m}$ we obtain
$$v=\sqrt{2 g L}=\sqrt{2\left(9.80 \mathrm{m} / \mathrm{s}^{2}\right)(1.20 \mathrm{m})}=4.85 \mathrm{m} / \mathrm{s}$$
