anybody use Giancoli physics 6th ed. I need help with a chpt 16 question: #12?

Question

particles of charge +75,+48 and -85 uC are placed in a line. The center one is 0.35m from each of the others. Calculate the net force on each charge due to the other two.
I actually have the solution, but i dont know where they get r=12, r=23 and r=13? and that "r" part is what i dont understand
PLEASE HELP !

Answer 1

I DON'T HAVE THE BOOK SO
(since the principal is the same)
Let
q1=+75,
q2=+48
q3= -85 uC
let q2 be in the middle
F=kq1q2/R^2

F2=-F12+F23
F2=k(-q1q2/R12^2 + q2q3/R23^2 ) since R12=R23
R12- distance between charge 1 and charge 2
R23 - distance between charge 2 and charge 3
F2=k/R^2(-q1q2+q2q3)
The rest is arithmetic
Does that help?