Electric charge and force question

[MRoach]

Homework Statement

q1 = 25 μC at (0,1) and q2 = 20 μC find a.) q3 if q1 is in the negative x direction

Homework Equations

E=kq1q2/r^2

The Attempt at a Solution

I keep trying to add vectors to find q3 ie ( -(25 μC) + 20 μC) and get 5 μC the book answer says 20 μC

 

[MRoach]

the force on q1 points in the -x direction