Reading the graph, you get 300 km in 4 hours.
Givens
Formula
v = d/t
Solution
v = 300/4 = 75 km/hour which is the slope.
Comment
Any point that you can read accurately will give you the same answer. Since the point are not that readable, you can estimate one of them
Try
t = 6 hours
d = 450 km An estimate. It looks to be 1/2 way between 400 and 500 km when t = 6 hours.
v = ??
v = 450/6
v = 75 miles / hour << Answer