You're supposed to arange the numbers in order from least to greatest then you mark out the highest and the lowest number. You keep this up until either there is one or two numbers left. In this case, there are two numbers as the median. Add $10 and $15 then you divide it by two.
Since they're both negative numbers and it went from -9 to -1, you had to have added a positive number to get a lesser negative number. To put it simply, if you were to subtract 8 from 9, you'd get 1.
