aqa a level mechanics Newton`s law help

Q: A particle of mass 3kg is sliding in a straight line across a smooth horizontal surface at a speed of 30ms^-1. It encounters a rough patch of length 50m, where the coefficient of friction between the surface and the particle is 0.4. Calculate the deceleration of the particle as it moves over the rough patch.
Since the object is originally sliding and has no acceleration, does that mean that there is no driving force?

What do you mean by a 'driving force' - its on a smoth surface to start with so it just travels at constant speed.

You need to work out the deceleratioon when it hits the rough patch. Draw a force diagram.

I have its just that once it hits the rough patch, it has a friction force of 11.76, but is that its overall force? menaing that -11.76=3a since f=ma?