3. Electrons repel each other with a force which is inversely proportional to the
square of the distance between them; call the proportionality constant k in the
units to be used. Suppose one electron is ﬁxed at x=0 on the x−axis.

Find the work done in moving the second electron along the x-axis from the point x=M to x=1.

Is is just integral from 1 to M of k/d^2 dx where d=M-1 or is this wrong?

2. 1 to M or M to 1? Make up your mind which way it is going.

Work = Force * Distance

Incremental Work = Instantaneous Force * $\Delta$Distance

I think you have it.