Question:

Two charges of magnitudes -2Q and +Q are located at points (a,0) and (4a,0) respectively. What is the electric flux due to these charges through a sphere of radius 3a with its centre at the origin?

By Gauss's law, the flux Φ = Qen/ε0
Here the charge enclosed by sphere of radius 3a is Qen = -2Q
Thus, Φ = -2Q/ε0