A particle moving eastward with a velocity of 5m/s.in 10 second the velocity changes to 5m/s

you have not mentioned the direction of the final velocity...I think that way qs is incomplete.suppose the final veloccity is southwards.

then,

change in velocity=v2-v1 in south west direction

magnitude of change in velocity=?(5²+5²-2x5x5 cos90)

                                             =5?2m/s