How do i find displacement on a position time graph ?

Displacement is just the distance between the start-point and the end-point,
regardless of the route that was traveled on the way.

If you're asked to find a displacement, then the first thing the asker has to do
is define the part of the graph that he's interested in.The easiest way to do that
is to specify the start-time and end-time.  Once you have those, the displacement
is easy.  Just look at the graph.  Find the position at the end, and the position
at the beginning.  Subtract the beginning position from the ending position, and
that's the displacement. 

Ignore everything in between.  No matter how many curves, curls, breaks, or
stops the graph may have on the way, you don't care.  The displacement is just
the difference between the end position and the start position.