2 questions

[Link]

A +5.7 microC and a -3.5 microC charge are placed 25cm apart. Where can a third charge be placed so that it experiences no net force?

I dunno how the book got 0.91m beyond the negative charge, I think its impossible to calculate since the magnitude of the third charge is not given.

[Nylex]

You don't need the magnitude of the third charge, as it will cancel.

Write down an expression for the force between the positive charge and the 3rd charge and another expression for the force between the negative charge and the 3rd charge. What can you say if there's no net force on the 3rd charge?

[Curious3141]

A +5.7 microC and a -3.5 microC charge are placed 25cm apart. Where can a third charge be placed so that it experiences no net force?

I dunno how the book got 0.91m beyond the negative charge, I think its impossible to calculate since the magnitude of the third charge is not given.

The point at which a charge experiences no net force is the point at which the electric field strength is zero. Find that point. You don't need the magnitude of the third charge.