\(F =\frac{ kq ^{2}}{( d + x )^{2}}-\frac{ kq ^{2}}{( d - x )^{2}}\)

\(F =- kq ^{2}\left[\frac{4 dx }{\left( d ^{2}- x ^{2}\right)^{2}}\right]\)

\(a =-\frac{4 kq ^{2} d }{ m }\left(\frac{ x }{ d ^{4}}\right)\)

\(a =-\left(\frac{4 kq ^{2}}{ md ^{3}}\right) x\)

So, angular frequency

\(\omega=\sqrt{\frac{4 kq ^{2}}{ md ^{3}}}\)

\(\omega=\sqrt{\frac{4 \times 9 \times 10^{9} \times 10}{1 \times 10^{-6} \times 1^{3}}}\)

\(\omega=6 \times 10^{8}\; rad / sec\)