Question

a gaussian surface contains three charge (-q),(+2q) and (-q). Evaluate net electric flux through the surface.

Answer 1

According to Gauss Law : electric flux is the ratio of net charge inclosed inside the gaussian surface to permittivity of medium.
$e.g., \bold{\phi =\frac{Q_{net}}{\epsilon_0}}$
where Φ is electric flux inclosed inside the gaussian surface ,ε₀ is the permittivity of medium { vacuum or air if medium doesn't mention }

here you can see in figure ,
Qnet = sum of all charge inclosed inside the gaussian surface
Qnet = -q -q + 2q = 0

$then, \bold{\phi=\frac{0}{\epsilon_0 } = 0}$
hence, electric flux = 0

Answer 2

Qnet = sum of all charge inclosed inside the gaussian surface  

Qnet = -q -q + 2q = 0  

then, \bold{\phi=\frac{0}{\epsilon_0 } = 0}

hence, electric flux = 0