When looking at a displacement-time graph, automatically you should realize that the slope is the instantaneous velocity at a specific point. That is, if x increases from 0 to 1 in 2 seconds, the slope would be the change in x divided by the change in time, so (1-0)/2 = 1/2.
As for your problem, we’re looking at when t=8.5s. At this point in the graph, we see a slope that starts at t=8 and ends at t=9, for the change in t of (9-8)=1 second. As for the change in x, we see it goes from -2 to 0 in this time for a change in x of (0-(-2))=2 (remember that a change in a variable is the final number minus the initial number). Therefore, the change in x over the change in time when t=8.5 seconds is 2/1 or just 2. So the instantaneous velocity at t=8.5 is 2m/s.