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 $\sqrt{11}N{C}^{-2}$. 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.