Find Minimum Potential of Two Charges on x-Axis

[eltel2910]

A charge of 0.883 nC is placed at the origin. Another charge of 0.347 nC is placed at x1 = 13.1 cm on the x-axis.

At which point on the x-axis does this potential have a minimum?

I do know a few things:

A necessary condition for the potential to have a minimum is that its derivative is 0.

The minimum is between the two charges.

I need to take the derivative of:
k*q1/r - k*q2/(r-x1), but I'm not sure about how to go about that?

I'm not looking for the answer. Just need some help setting this up.

 

[andrevdh]

There should be a point in between the two charges where a test charge experiences no force at all. Which means that you could just plonk the test charge in there. No need to do any work to get it there, you could just slip it in sideways!