A ball A is thrown up vertically with a speed u and at the same instant another ball B is released from a height h. At time t, the speed of A relative to B is

At time t

Velocity of A, $v_A=u-gtupward$

Velocity of B,  $v_B=u-gtdownward$

If we assume that height h is smaller than or equal to maximum height reached by A then at every instant  $v_{A}and{v}_B$ are in opposite direction.

$\therefore v_{AB}=v_A+v_B=u-gt+gt$

(speeds in opposite directions get added)