You use Newton's second law, which says that the net force acting on a body is equal to the product of the body's mass and the acceleration it undergoes. That is,
[tex]\sum\vec F=m\vec a[/tex]
From the question, it sounds like the given force is the only force acting on the block, which means there is only going to be one direction of movement if the acceleration is constant. This allows us to ignore the vector notation and talk only in terms of magnitudes. The above equation reduces to
[tex]F=ma[/tex]
So in this case,
[tex]F=ma\implies a=\dfrac Fm[/tex]
and numerically, we get
[tex]a=\dfrac{40\,\rm N}{12.5\,\rm kg}=3.2\,\dfrac{\rm m}{\mathrm s^2}[/tex]
