The mode is the value that occurs most often. The numbers you were given include two 6's, two 11'a, and two 12's. So to make 6 the most common element of this set there must be at least three of them. So that's one of the unknowns.
The median is the middle value when you put them all in order. There are 11 numbers in this set, so the median is the 6th number listed in order. Hence 7 must be the 6th number.