Force is the product of the mass of the body on which it is applied multiplied by the acceleration produced. One Newton of force accelerates a body of 1 kilogram mass by 1 m/s^2.
In the question asked, the force applied produces an acceleration of 5.7 m/s^2 on a body weighing 2.1 kilograms. So the force applied is 2.1* 5.7 = 11.97 N
When the mass of the body is increased to 10.3 kg, the same force of 11.97 N produces an acceleration of 11.97 / 10.3 = 1.16 m/s^2.
The required acceleration produced is 1.16 m/s^2
Force applied is given by:
Force = Mass x Acceleration
Force applied in the first case = 2.1 x 5.7 = 11.97 N
Force applied in sccond case = 10.3 x (Acceleration in second case)
As the force applied is same in both the cases:
11.97 = 10.3 x (Acceleration in second case)
Accelerationin second case = 11.97/10.3 = 1.1621 m/s^2
Acceleration = 1.1621 m/s^2