Electromagnetic forces, finding alpha.

[sphys4]

Homework Statement

When a third charge is placed at the origin, it is found that the force on it is zero. Find alpha?

Two charges Q and 5Q are located as shown in the figure below.
https://chip.physics.purdue.edu/protected/GiordanoEMimg/chapter17/17prob16.jpg



If possible, please show work so i can learn how to do it. Thanks!

Homework Equations

The Attempt at a Solution

I know that force varies as a function of charge and inversely proportional to square of distance.
So i need to use this equation somehow:
F = k Qa Qb / d^2

 

[ideasrule]

That link can only be accessed by Purdue University students. Can you attach the image instead?

 

[sphys4]

 

[gneill]

So suppose that some charge, q, was placed at the origin. What would be the formula for the force on it due to the charge Q? (And for the sake of argument, suppose for now that all the charges, q, Q, and 5Q are positive).