Net flux through the cylindrical surface is given as
[tex]\phi = \frac{q}{epsilon_0}[/tex]
here q = enclosed charge in the surface
so here in order to find the value of q
[tex]q = \lambda* L[/tex]
so now we have
[tex]\phi = \frac{\lambda * L}{\epsilon_0}[/tex]
so this is the total flux
now by Gauss's law we can find the electric field
[tex]\int E.dA = \phi[/tex]
[tex]\int E.dA = \frac{\lambda * L}{\epsilon_0}[/tex]
[tex]E* 2\pi rL = \frac{\lambda * L}{epsilon_0}[/tex]
[tex]E = \frac{\lambda}{2\pi \epsilon_0 r}[/tex]
by above expression we can find the electric field at required position