From Mechanics

The kinetic energy of the particle at \displaystyle A is converted to potential energy at \displaystyle B.

Using the result of part a)

\displaystyle \begin{align} & 0\textrm{.}036=0.05\times 9\textrm{.}8\times (0\textrm{.}8-0\textrm{.}8\cos \theta ) \\ & \cos \theta =0\textrm{.}908 \\ & \theta =24\textrm{.}7{}^\circ \\ \end{align}