Let s be the plane's speed in still air and w the wind speed. Then
s - w = (6060 km)/(6 h) = 1010 km/h
s + w = (12,150 km)/(9 h) = 1350 km/h
Adding these equations together eliminates w and lets you solve for s :
(s - w) + (s + w) = 1010 km/h + 1350 km/h
2s = 2360 km/h
s = 1180 km/h