Chapter 2 - Kinematics in One Dimension - Exercises and Problems - Page 64: 13

$a= 8.75ms^{-2}$

We have initial velocity $u=300m/s$ final velocity $v=400m/s$ distance travelled in between change of velocity $s=4.0km= 4000m$ We know that $2as= v^2 - u^2$ $\therefore \space a =\frac{ v^2 - u^2}{2s}$ $ a = \frac{400^2 - 300^2}{2*4000} ms^{-2}$ $a = \frac{160000- 90000}{8000}ms^{-2}$ $a=\frac{70}{8} ms^{-2}$ $a= 8.75ms^{-2}$