How Do You Calculate the Unknown Charge in an Electrostatic Field?

[Nitrate]

Homework Statement

An unknown charge and a charge of 60 nC are 2.0m apart. The electric field strength is zero at a point 0.80m from the unknown charge on a line connecting the two charges. Find the magnitutde of the unknown charge.

Homework Equations

q=F/E
F=kq1q1/r^2
E=kq/r^2

Coulomb constant: 8.99 x10^9

The Attempt at a Solution

I tried making an equation with the info given, but it doesn't seem like it's going to help me:
0=(8.99x10^9)(x)/(0.8)^2

i think i just need a push in the right direction

 

[Nitrate]

anyone?

 

[JHamm]

What is the equation of an electric field with 2 charges present?

 

[Nitrate]

What is the equation of an electric field with 2 charges present?

uhh?
force?

 

[JHamm]

If the system has 2 charges how do you find the force on a third charge at a point? :)