Newton's second law states that:
[tex]F=ma[/tex]
i.e.: to accelerate an object with mass m with an acceleration of a, we must apply a force of intensity F.
In our problem, the block has a mass of m=12.5 kg, and its acceleration is [tex]a=3.5 m/s^2[/tex], so the force applied is
[tex]F=(12.5 kg)(3.5 m/s^2)=43.74 N[/tex]
