The centroid is the point where the 3 medians meet. In the diagram, the centroid is M. A median is a line that joins the midpoint of the side opposite the angle opposite the midpoint. RC is the median of PQ. You might have to read this a couple of times to have it make sense.
To find RM, you have to realize that the centroid cuts the median line in a ratio of 1:2.
In this problem, you are given more information than you need. The median containing RM is RC. If CM is 7, RM will be twice as long which is 14.
