The net applied force responsible for the acceleration is 15.3 N.
According to Newton's second law of motion, the net unbalanced force acting on any object is directly proportional to the product of mass and acceleration acting on that object. So
As the mass of the object is given as 5.1 kg and the acceleration attained by the object or scooter is 3 , then the net applied force on the scooter is
Thus, the net applied force responsible for the acceleration is 15.3 N.