From Mechanics

We first obtain the acceleration \displaystyle a.

Using the kinematic equation (see section 6)

\displaystyle {{v}^{\ 2}}={{u}^{\ 2}}+2as

\displaystyle \begin{align} & {{0}^{2}}={{50}^{2}}+2\times a\times 300 \\ & \\ & a=-\frac{2500}{600}=-4.167\ \text{m}{{\text{s}}^{-2}} \\ \end{align}