For the first one, I'm not really sure but I think that this is how to do it.
Imagine a graph of the velocity of the object.
Its velocity is 4 m/s at x = 0, and its velocity is 0 m/s at x=12 meters.
So if y is the velocity and x is the distance, the equation of that line is: y = (-4/12)x + 4, or
y = (-1/3)x + 4.
The derivative of velocity is acceleration, so the acceleration is -(1/3) m/s/s.
You know the acceleration is going to be negative because the object is slowing down.
I'm not sure if this is right though because I didn't use the information about it being 2 kg at all.