Where the electric field is zero

[JJones_86]

1. The problem statement, all variables and given/known data
Two charges, -20.6 microC and +4.2 microC, are fixed in place and separated by 3.3 m.

(a) At what spot along a line through the charges is the net electric field zero? Locate this spot relative to the position of the positive charge.

(b) What would be the magnitude of the force on a third charge +24.1 microC placed at this spot?


2. Relevant equations

F = k (q1*q2)/r^2

3. The attempt at a solution

None

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I've got part A to be 2.71679m from the positive charge, I just don't know how to do part B. The equation I posted seems to me irrelevant because there are now 3 charges. Any help is appreciated.
1. The problem statement, all variables and given/known data



2. Relevant equations



3. The attempt at a solution

[rl.bhat]

(b) What would be the magnitude of the force on a third charge +24.1 microC placed at this spot?
Find the force between +24.1 microC and+ 4.2 micro C. Force due to other charge will be equal and opposite to this force.

[JJones_86]

(b) What would be the magnitude of the force on a third charge +24.1 microC placed at this spot?
Find the force between +24.1 microC and+ 4.2 micro C. Force due to other charge will be equal and opposite to this force.

F = k*q1*q2/r^2
F = 8.99E9(4.2E-6)(24.1E-6)/2.71679^2
F = 0.123286 N?

[JJones_86]

(b) What would be the magnitude of the force on a third charge +24.1 microC placed at this spot?
Find the force between +24.1 microC and+ 4.2 micro C. Force due to other charge will be equal and opposite to this force.

I'm not exactly sure what you mean by equal and opposite, would it make it 0 N then?