Velu, as you can see in the text it says “but looking at dendrogram from H-clust below we see that luminal could be split into two clusters, which correspond to our two clusters here).” – the dendrogram is an output from H-clust (hierarchical clustering). K-means clusters can be visualized using several methods like PCA where we have a scatterplot, but the additional visualization done by coloring samples that belong to a cluster with color associated with K-means output. We will have such a tutorial available soon as a standalone lesson.