Small clarification: applying Coulomb's Law

1. The problem statement, all variables and given/known data
Hi,

I'm studying electric charges and fields, and I have a quick question about this example in my book:


Everything is straightforward except the subscripts (13 and 23) on r in the denominators of initial Coulomb equations for F₁ and F₂. In the next step they just plugged in the distances, so those numbers don't seem to do anything, and there are no subscripts in the original formula. So what are they?


2. Relevant equations

F = k [(q₁q₂)/r²]

 

Duh, I knew it had to be something obvious. Thank you!