A particle moves in a straight line relative to an origin $O$. The displacement, $s$ metres, of the particle from $O$ at time $t$ seconds is given by

$$s=p+5t-k{e}^{-0.4t}$$

where $p$ and $k$ are constants. At time $t=2$, the acceleration of the particle is $-2.4{m\; s}^{-2}$.