To solve this problem, we have to use the electric field definition.
[tex]E=K\cdot\frac{q}{r^2}[/tex]Replacing the needed information, we have
[tex]3370=9\times10^9\cdot\frac{q}{(0.5)^2}[/tex]Let's solve for q
[tex]\begin{gathered} q=\frac{3370\cdot0.25}{9\times10^9} \\ q=93.6\times10^{-9}C \\ q=9.36\times10^{-8}C \end{gathered}[/tex]