it seems obvious to me that this is for an assignment or something, so i won't give you full solutions. that would be cheating. anyway
now, lets say they pass each other at a distance x from city 1. so the 800km/h plane travels x km. that would mean the 900km/h plane traveled (2550 - x) km.
so, for the 800-plane, time = x/800
for the 900-plane, time = (2550 - x)/900
this is a pair of simultaneous equations with two unknowns. you can solve for time
do that while i work on the others.