$S_{ rel }=U_{ rel } t+\frac{1}{2} a_{ rel } t^2$
$\Rightarrow S_{ rel }=(60-40) 5 \quad\left(a_{ rel }=0\right)$
$\Rightarrow S_{ rel }=100 \,m$
$\begin{array}{|c|c|c|c|c|} \hline t( s ) & 0 & 1 & 2 & 3 \\ \hline x ( m ) & -2 & 0 & 6 & 16 \\ \hline \end{array} $