Clustering

Let the set $D=\{18,11,8,10,5,2,20\}$ We want to redistribution the data of $D$ into two $(2)$ clusters, using the K$-$means algorithm. The distance $d$ between two numbers $a$&$b$ is calculated as follows: $d(a,b)=|a-b|($the absolute value of a minus b$).$

a$-$ Apply $K-$means choosing as initial centers of the $3$ clusters respectively: $8$ and $11.$ Show all the calculation steps. $(9$ pts$.)$

b$-$ When do we use $k-$means? $(1$ pts$.)$