\displaystyle \mathbf{a}=\frac{\mathbf{F}}{4}= \frac{\smash{ {12t\mathbf{i}+(6-5t)\mathbf{j}} }}{4}= 3t\mathbf{i}+\left( \frac{3}{2}-\frac{5t}{4} \right)\mathbf{j}
