How much work must be done to bring three electrons from a great distance apart to within 2.1×10−10 {\rm m} from one another (at the corners of an equilateral triangle)?
Let a be the separation. I do not understand your 1×10−10 {\rm } notation for a
Bringing in the first electron requires no work.
Bringing in the second electron required k^e^2/a
Bringing in the third electron requires
2ke^2/a
The total work required is
3 k e^2/a
k is the Coulomb constanmt