15 must be added. the median is the middle number of the set, if it is an even amount in the set the two middle are averaged.
9, 12, 13, 15, 17
the median here is 13, but we need it to be 14. so if we add 15.
9, 12, 13, 15, 15, 17
the two middle numbers are 13 and 15.
to average we add and divide.
13+15=28/2=14 :)