This has something to do with graphing (speed time graph) so I suspect this is the right forum.

Question: Two stations A and B are 495m apart. A train starts from A and accelerates uniformly for 15s to a speed of which it maintains until it is 90m from B. Sketch a speed-time graph to show the motion of the train. Find the magnitude of the acceleration and retardation and the total time the journey takes.

I'm stuck on the magnitude of acceleration =\ .