Initially sled is moving with uniform velocity which means the net force must be balanced on the sled
[tex]F_{applied} = F_{friction}[/tex]
So it is given that applied force on the sled is 200 N and sled is not accelerating so friction force must be 200 N.
[tex]200 N = F_{friction}[/tex]
Now suddenly it starts with 600 N applied force due to which it will accelerate and this can calculated by Newton's II law
[tex]F_{applied} - F_{friction} = ma[/tex]
[tex]600 - 200 = \frac{400}{g} *a[/tex]
[tex]400 = 40 * a[/tex]
[tex]a = 10 m/s^2[/tex]
So in this case the sled ill start accelerating with acceleration [tex]a = 10 m/s^2[/tex]
