Hello
To solve this question, let's write out the data given.
[tex]\begin{gathered} f=12N \\ a=3ms^{-2} \\ m=\text{?} \end{gathered}[/tex]The formula for newtonian force is given as
[tex]\begin{gathered} f=ma \\ 12=m\times3 \\ 12=3m \\ \text{divide both sides by the coefficient of m} \\ \frac{12}{3}=\frac{3m}{3} \\ m=4\operatorname{kg} \end{gathered}[/tex]From the calculations above, the mass of the object 4kg