Analysis question from Baby Rudin (PMA), Chapter 1

This is number 19 from Chapter 1 of "Principles of Mathematical Analysis." It's really confusing to me. Here we go:

Suppose **a** belongs to R^k, and **b** belongs to R^k. (Where R^k is a Euclidean Space) Find **c** in R^k and r>0 such that

|**x **-** a**| = 2|**x** - **b**|

if and only if |**x **- **c**| = r.

Solution: 3**c** = 4**b** - **a**, 3r = 2|**b** - **a**|.

I've put the "vectors" in bold type. Also the "solution" is given in the book. I'm not sure how we're supposed to use it... whether or not we simply plug it in and show that it works, or whatever...

I am very confused about this question. Please help! Thanks.