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Calculating frequency of oscillation

> A 4C point charge of mass 2kg is suspended by a string of length 6m. The charge is placed in the Earth's Gravitational field and a uniform horizontal electric field of strength $\sqrt{11}NC^{-2}$. If the charge is displaced slightly from equilibrium, what will be the frequency of oscillation? (g=10m/s)

I was trying to solve the question following way.


$$f=\frac{1}{2\pi} \sqrt{\frac{k}{m}}$$


$$f=\frac{1}{2\pi} \sqrt{\frac{F}{ml}}$$


$$f=\frac{1}{2\pi} \sqrt{\frac{Eq+mg}{ml}}$$

Where, $m$ is mass. $l$ is length. $E$ is electric field. $q$ is point charge. $g$ is gravitational acceleration. But, my answer was wrong. What am I missing?

The correct answer is $$\frac{\sqrt{2}}{2\pi}$$ 

But, I couldn't solve it anyway.