The slope-intercept form: y = mx + b
(m - slope, b - y-intercept)
We have y = -2. It's the slope intercept form where m = 0 and b = -2.
y = -2 → y = 0x - 2
Uh, so, you're median is 81 already.
Since the middle two numbers are 82 and 80, you have to add them together and then divide by 2.
However, if you're trying to add a number in, adding 81 as it is would work as well.
Hope this helps!