Heya!!
For calculate aceleration, let's applicate second law of Newton:
[tex]\boxed{F=ma}[/tex]
⇒ Being:
→ F = Force = 12 N
→ m = Mass = 3 kg
→ a = aceleration = ?
Lets replace according formula and leave the "a" alone:
[tex]12\ N = 3\ kg * \textbf{a}[/tex]
[tex]\textbf{a} = 12\ N / 3\ kg[/tex]
[tex]\textbf{a} = 4\ m/s^{2}[/tex]
Result:
The aceleration of the object is of 4 m/s²