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## Homework Statement

Charge 8e-18 C is on the y axis a distance 2 m from the origin and charge

9e-18 C is on the x axis a distance d from the origin. The Coulomb constant is 8.98755e9 Nm^2/C^2.

What is the value of d for which the x component of the force on 9e-18 C is the greatest?

## Homework Equations

Coulomb's law: F = kq1q2/r^2

## The Attempt at a Solution

I tried to use Coulomb's law to find the net force and then to find the force in the x direction but I became very stuck.

F = 8.98755e9 Nm^2/C^2 * 8e-18 C * 9e-18 C / (4 + d^2)

My problem is I have two unknowns and I can't find the value of d. Please help if you can.