An electrostatics lab activity uses three charged spheres (that can be modeled as point charges) to study electric forces. Spheres s₃ is placed at the origin, s₂ at x = 4.00 cm, while s₁ is placed at 8.00 cm. Spheres s₃ and s₁ have charges -4.00 nC and 6.00 nC, respectively. What is the magnitude and sign of the charge s₂ if the net force from the other charges on s₁ is zero?