Ah ok! It was important because we know that $\displaystyle v_0=0$ and even more important : we know that the initial average acceleration phase lasts 3 seconds.
They say
Using the formula Vf=Vi+at, calculate the velocity of the cars at the end of their initial average acceleration phase.

but I'd like to precise a bit more the formula into this one : $\displaystyle v_{final}=v_{initial}+a\cdot t_{final}=a \cdot (3s)=4.44\frac{m}{s^2}\cdot 3s=13.33\frac{m}{s}$ which is the answer.
In fact the problem just asked you to plug values into the given formulas.