Electric fields caused by multiple charges

[lacar213]

Homework Statement

A particle with charge +7.88 μC is placed at the fixed position x = 3.00 m in an electric field of uniform strength 300 N/C, directed in the positive x direction. Find the position on the x-axis where the electric field strength of the resulting configuration is zero.

Homework Equations

E= K*Q / r^2

The Attempt at a Solution

I set up the equation with E = 0 but I'm not sure how you can solve it now for R since e = 0

 

[Dadface]

Find a value for r such that E becomes equal and opposite to 300N/C ie a point where the 2 fields cancel to zero.