Well, part of 'velocity' is the direction of the motion, and the graph doesn't give us any information about the direction. So the best we'll be able to do is find the average SPEED of the car.
On a distance/time graph, the speed is just the slope of the line. Since this graph is a straight line, it doesn't matter which points along the line you choose ... the answer will always be the same.
Average speed = (distance covered) / (time to cover the distance)
Let's use the ends of the line ... the origin, and the point at the top.
On the left side of the graph, we see the label 'Distance (m)', and I don't know what 'm' stands for. It could be 'miles' or 'meters'.
Anyway, the point at the top of the line shows that the car covered a distance of 25.0 'm' in 10.0 s of time.
Average speed = (25 m) / (10 s)
Average speed = (25/10) m/s
(I'm pretty sure that 'm' means 'meters' and 's' means 'seconds'.)
