$u_{\text {rel }}=u_1-u_2=0-(-5)=5 ms ^{-1}$
$t=3 \,s$
$a_{ rel }=a_1-a_2=-g-(-g)=0 \,ms ^{-2}$
$s_{ rel }=u_{ rel } t+\frac{1}{2} a_{ rel } t^2$
$\Rightarrow s_{ rel }=5 \times 3=15 \,m \quad\left(\because a_{ rel }=0\right)$
So, $s_{\text {rel }}=15 \,m$