If you leave 9 minutes after your parents, you have to travel the same distance in 9 minutes less time so since distance is speed times time, 67t=73(t-9/60), where t is time converted to hours.
67t=73t-3*73/20=73t-219/20; 73t-67t=10.95; t=10.95/6=1.825 or 1hr 49.5 min after 11:30=13:19.5.