How to derive magnitude of Electric field to this?

[Innuendo]

[Ignore]How to derive magnitude of Electric field to this?

Homework Statement

My book states that
$$|E| = \frac{2kQ}{d^2} = 2\pi k\sigma$$
I thought k, the coulomb constant is $$\frac{1}{4\pi\epsilon0}$$

Homework Equations

The Attempt at a Solution

 

[segiterrus]

k ≈ 8.854 187 817 x 10-12 F m-1