Consider plane A with respect to B,
initial relative Altitude 
relative rate of altitude(speed) $r_{a,b}= 55.75-35.5=20.25$
To get at same altitude, they'll take same time, and their relative height will be $0$ So,
$H_{a,b}=h_{a,b}+r_{a,b}t$
$0=-972+20.25t$
$t=48$
and altitude will be, $3028+55.75\cdot 48=5704$