Power =(force)(velocity) At x=0 , v=1 , P=2, F=(10/3)a Hence $2=(10/3)a(1)$ $a=(14/10)$ Now at x=10 , P=4 $4=(10/3)(14/10)v$ $v=2$ at x =10 But it is wrong why ?

You are assuming that force and acceleration are constant. They are not constant. So acceleration at $x=10$ is not equal to that at $x=0$.