What is the Optimal Distance d for Maximum x-Component Force on Charge q?

[roj_mah]

Homework Statement

Charge Q is on the y-axis a distance a from
the origin and charge q is on the x-axis a
distance d from the origin.
What is the value of d for which the x
component of the force on q is the greatest?

Homework Equations

The Attempt at a Solution

 

[minimark1234]

can you show your work so maybe we can see where you went wrong, and we could try to help you from there?