Answer
$$\dfrac {50\times 10^{-3}\times \left( 8\times \cos 30\right) ^{2}}{2}=1.2J$$
Work Step by Step
At the top of its flight ball will only fly with horizontal component of its velocity so
$E_{k}=\dfrac {mv^{2}_{x}}{2}=\dfrac {m\left( v_{0}\times \cos \alpha \right) ^{2}}{2}=\dfrac {50\times 10^{-3}\times \left( 8\times \cos 30\right) ^{2}}{2}$
$$\dfrac {50\times 10^{-3}\times \left( 8\times \cos 30\right) ^{2}}{2}=1.2J$$
