Displacement on a Position vs. Time Graph

1. The problem statement, all variables and given/known data
The graph is of a runner-
time(min) is the x
position(x1000 m) is the y
these are the points
0,0
10,3
20,3.5
30,4.5

The question is- For the time interval between 10 min and 20
min, what is the runner’s displacement?
Answer in units of m



2. Relevant equations



3. The attempt at a solution
The position for 10 min is 3000 and the position for 20 is 3500. I subtracted 3500 from 3000 to get 500, what i thought would be the displacement. This is not the answer however and i don't know any other way to work out this problem. Help is appreciated, thanks!

 

Turns out it was 20,3.4 and not 3.5. Thanks though