What is the electric field strength at the center of the triangle?

[Mozart]

Homework Statement

What is the electric field strength at the center of the triangle?

Three 13.0 -cm-long rods form an equilateral triangle. Two of the rods are charged to + 19.0 nC, the third to - 19.0 nC .

What is the electric field strength at the center of the triangle?

Homework Equations

E=(kq)/r^2 lambda=deltaQ/deltax

The Attempt at a Solution

I've attempted this problem about 4 times and got all different but wrong answers. In each attempt I figured that the electric fields created by the two positively charged rods cancel each other out everywhere (This is where I believe I have made my mistake) and that the electric field depends only on the negative charged rod. I also figured by symmetry that the components of the electric field to the left and right (x-axis I chose) cancel each other out and the only electric field felt at the center is downwards (Y-axis I chose).

Is my mistake based on that I figured the two positively charged rods cancel each others electric fields out? I'm looking for some tips and hints to get me on track for this problem.

Thank you.

 

[Hootenanny]

Okay, let's start with the expression for the electric field of a finite line of charge.

 

[Mozart]

is it E line=(K)(2)absolutevalue(lambda)/r ?

=(k)(2)absval(Q/x)/r

 

[Hootenanny]

That is for an infinite line of charge, what about a finite one, any ideas?