Ijust don't know how to do the question (a) and (b)

Using the equation

we can observe that you have to apply a non-zero net force to an object in order to make it accelerate. In fact, if the net force is zero you have

Since we're assuming

Now, if the 12N force is applied, the object moves with a constant speed. A constant speed means no acceleration, since by definition the acceleration is a change in speed.

If this sounds counterintuitive to you (why I'm applying a force but I have to acceleration?) think of when we drive a car: even if you want to keep your speed constant, you still have to use the gas pedal, just enough so that the push of the motor balances exactly the road/wheels friction. If you give less gas, the friction becomes stronger, and the car slows down. If you give more gas, the motor push becomes stronger, and the car accelerates.

Back to your exercise: constant speed means to acceleration, so the net force must be zero. This implies that the friction force is exactly 12N.

If the force is increased to 18N, there will be a net force of 6N pushing the object, causing it to accelerate. Using again the same equation of before, and plugging the 3kg mass in the equation, we have

So, the object moves with constant acceleration and initial speed of 10m/s for 0.2 seconds. It's final speed will be

5 m/s

Explination:In fact, the ball's average velocity during its second of fall is 5 m/s (= (0 m/s + 10 m/s)/2). For half of the second the ball has been going less than 5 m/s, and for half of the second the ball's speed has been more than 5 m/s.