The net force is what remains of the pull when we subtract the friction force:

Now, use the law

and solve it for the acceleration

to get the result:

<span>f(x) = ax2+bx+c, is quadratic equation
</span><span>function opening downward if the a<0,
</span><span>kf(x) = -x², a= -1<0
so the answer is </span><span>B.kf(x) </span><span>
</span>