Answer:
[tex]given \\ accelaration (a)= 2.5 \frac{m}{ {s}^{2} } \\ initial \: velocity(u) = 21 \frac{m}{s} \\ final \: velocity(v) = 31 \frac{m}{s} \\ we \: want \: find \: time \\ newtons \: equation \: of \: motion \: v = u + at \\ 31 = 21 + 2.5 \times t \\ 31 - 21 = 2.5t \\ 10 = 2.5t \\ \frac{10}{2.5} = t \\ t = 4sec[/tex]