Jackson Classical Electrodynamics: Deriving W_{int} from (1.57) to (1.58)

[americanforest]

Can someone explain to me how Jackson, on page 42 of the 3rd ed. of Classical Electrodynamics, when he is deriving the interaction energy W_{int} in his example involving two point charges, gets from equation (1.57) to (1.58). I thought about typing up the TeX but I'm sure most of you have this book. I know he makes that substitution he mentions but I'm not sure how to go about doing that. Help!

 

[Born2bwire]

I keep my copy of Jackson at the office and not at home. I do not need him to pervade my life any more than is possible.

 

[Redbelly98]

Some of us are old and have earlier editions of Jackson. Presumably the equation numbering is different.

 

[gabbagabbahey]

It's just a straight substitution: Solve for \mathbf{x} in terms of \mathbf{\rho}, and substitute. If you get stuck, post your work and we'll be able to help you better.