Two spheres, solve for electric field strength

[axgalloway]

Homework Statement

Two 2.0-cm-diameter insulating spheres have a 6.50 cm space between them. One sphere is charged to + 89.0 nC, the other to - 95.0 nC.

What is the electric field strength at the midpoint between the two spheres?

Homework Equations

Esphere = KQ/r^2
k = 8.99 * 10^9

The Attempt at a Solution

Esphere1 + Esphere2 = Etotal

(k*89*10^-9) / (3.25*10^-2)^2 + (k*95*10^-9) / (3.25*10^-2)^2
= Wrong answer: 1.57*10^6 N/C

 

[Delphi51]

Nasty tricky wording!
Carefully sketch the spheres and mark all the distances clearly.
Remember the wording on that force formula: "distance between the centers of the charges".

 

[axgalloway]

Here was my 2nd attempt at a solution
Esphere1 + Esphere2 = Etotal

(k*89*10^-9) / (4.25*10^-2)^2 + (k*95*10^-9) / (4.25*10^-2)^2
= Wrong answer: 4.58*10^6 N/C

 

[axgalloway]

Here was my 2nd attempt at a solution
Esphere1 + Esphere2 = Etotal

(k*89*10^-9) / (4.25*10^-2)^2 + (k*95*10^-9) / (4.25*10^-2)^2
= Wrong answer: 4.58*10^6 N/C

Actually, that solution is a calculator error.
Here was my 3rd attempt at a solution (pressing the calculator correctly this time)
Esphere1 + Esphere2 = Etotal

(k*89*10^-9) / (4.25*10^-2)^2 + (k*95*10^-9) / (4.25*10^-2)^2
Correct answer = 9.16 * 10^5 N/C

Thanks for the help.