You can think of rate of change, as the slope of the graph.
For every unit you move in the x positive direction, how many units do you move in the y-direction?
Remember, slope is Rise/Run. So (change in y)/(change in x).
In your case it will be easier to see how many units you move in the y-direction per 6 units in the x-direction.
Looks like, if you move 6 units in the x-direction you will move 50 units in the y-direction.
Since we started at the origin we can compute:
Change in y is 50-0=50
Change in x is 6-0 = 6
Now applying: (change in y)/(change in x)
We have 50/6 = 25/3
So the rate of change for this graph is 25/3