To plot a velocity-time graph, put speed on the y axis and time on the x axis. Then plot the points you are given: in this case you know that the bike is initially at rest, is going 100km/h after 4 sec and 40km/h 20 seconds after it reached 100km/h. Since you are dealing with constant acceleration, you can just draw straight lines between the points.
Since you don't seem to have dealt with the equations of constant acceleration yet, I suggest using the fact that the distance travelled is the area under the velocity-time graph. Divide it up into shapes you can calculate the area for. (you may need Pythagoras' theorem. If you haven't learnt that yet as well as the equations of constant acceleration I dunno how you are meant to do this part.)