Answer
$$ v_{b}=2.42 \mathrm{m} / \mathrm{s}$$
Work Step by Step
In this case, the total mechanical energy is shared between kinetic $\frac{1}{2} m v_{b}^{2}$ and potential $m g y_{b} .$ We note that $y_{b}=2 r$ where $r=L-d=0.450 \mathrm{m} .$ Energy conservation leads to
$$
m g L=\frac{1}{2} m v_{b}^{2}+m g y_{b}
$$
$\text { which yields } v_{b}=\sqrt{2 g L-2 g [2r] ]}=2.42 \mathrm{m} / \mathrm{s}$
