From Mechanics

The loss in potential energy is equal to the gain in kinetic energy.

As the potential energy is zero when the particle is at its lowest position the kinetic energy is from part a)

\displaystyle \begin{align} & KE=52\textrm{.}5\text{ J} \\ & \text{52}\text{.5}=\frac{\text{1}}{\text{2}}\times 5\times {{v}^{2}} \\ & v=4\textrm{.}58\text{ m}{{\text{s}}^{\text{-1}}} \\ \end{align}