speed of the car at t = 10 s is given as 10 m/s
now at t = 30 s car comes to rest so v = 0
now in this velocity time graph we can say slope will show the acceleration of the car as we know that acceleration is rate of change in velocity
as we know that
now from above formula
So slope of the graph will represent the acceleration of car which is 0.5 m/s^2
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