The median of a set of values is the value that divides it into two equal subsets. One containing all the values that are less than the median, and another containing all the values that are greater than the median.
For a set with an odd number of values, the central value, when the set is written in ascending order, corresponds to the median.
For a set whit an even number of values, there are two central values (when written in ascending order). And the median corresponds to the mean of those two central values.
The given set is 7,-9,7,-9,-9,-1, which has an even number of values. So, after writing it in ascending order, we need to find the mean of the two central values:
-9, -9, -9, -1, 7, 7
The two central values are -9 and -1. And their mean is:
[-9 + (-1)]/2 = (-9 - 1)/2 = -10/2 = -5
Therefore, the median is -5.