The acceleration due to gravity for any object, including 1 washer on the string, is always assumed to be
m/s2.
The mass of 3 washers, when converted to kg, is
kg.
The applied force of 3 washers will increase the applied force on the car to
N.
The acceleration due to gravity is approximately 9.81 m/s^2.
The mass of 3 washers is typically 3 times the mass of 1 washer. If the mass of 1 washer is x kg, the mass of 3 washers would be 3x kg.
The applied force on the car is equal to the mass of the washers times the acceleration due to gravity, so the applied force of 3 washers would be 3x * 9.81 N.