Answer:
-3000
Step-by-step explanation:
The missing number is the slope of the line.
To calculate the slope, we use the formula

Using the points (0, 80000) and (10, 50000) [remember, the year 2000 is at x = 0; this means the year 2010 would be at x=10] we have:
m = (50000-80000)/(10-0) = -30000/10 = -3000
The missing number, the slope, is -3000.