tree that approximates the distances and total weight in graphs

I have this problem : given undirected graph we can build a tree that approximates the distances from given vertex ,r, and the total weight,
i.e. for every vertex x, $d_G(r,x) \le d_T(r,x) \le 3d_G(r,x)$ and : $w(T) \le 3MST(G)$ could any one help me to understand how to build this tree and if there are any material that would help ...

-