Resposta :
[tex]\boxed{\begin{array}{l}\huge\boxed{\boxed{\boxed{\boxed{\rm{a=\dfrac{\Delta V}{\Delta t}}}}}}\\\Large\displaystyle\text{$\begin{gathered}\rm -3=\dfrac{0-15}{t}\end{gathered}$}\\\Large\displaystyle\text{$\begin{gathered} \rm-3t=-15\end{gathered}$}\\\Large\displaystyle\text{$\begin{gathered}\rm t=5\,s~\checkmark\end{gathered}$}\end{array}}[/tex]