Answer: +2 e
Explanation: Net charge of a system is sum of charges on all the constituents of the system.
The system contains two spheres. Charge on one sphere is:
q₁ = +6 e
Charge on the second sphere is:
q₂ = -4 e
Net charge of the system is:
q = q₁ + q₂
q = +6 e + (-4 e) = + 2 e
Thus, the net charge of system is +2 e.