If the distance-time graph is a straight line then the motion is uniform. Time is given to. D v x t where v is the velocity average speed t is the time and d is distance so you can read it as Distance Speed x Time.
To find the speed of an object from a distance-time graph we need to calculate the gradient or slope of the line.
To calculate the speed select a triangular section of the graph. To calculate the speed select a triangular section of the graph. Use the formula speed distancetime to calculate the speed. First choose two points that lie the line - say point A is 0 0 and point B is 10 50.