Calculating Electric Field and Force Between Charged Rings

[littlejon]

Homework Statement

Two 10-cm-diameter charged rings face each other, 25cm apart. The left ring is charged to -17nC and the right ring is charged to +17nC .
A.What is the magnitude of the electric field E⃗ at the midpoint between the two rings?B.What is the magnitude of the force F⃗ on a -1.0 nC charge placed at the midpoint?
Express your answer to two significant figures and include the appropriate units.

Homework Equations

(E1)z=ZQ1/4πε0(z^2+r^2)^(3/2)

The Attempt at a Solution

((9E9)(.10)(-17E-9))/((.10)^2+(.50)^2)^3/2= 2.2E4.
Unfortunately
this is not the answer any help?

 

[haruspex]

You don't seem to have correctly matched up the z and r variables in the equation to the given numbers. What values for z and r have you used?