http://www.google.co.in/imgres?q=north+east+west+south&start=18&num=10&hl=en&gbv=2&biw=1024&bih=653&tbm=isch&tbnid=EUQqMvxPdiWwHM:&imgrefurl=http://ugconlinecoaching.blogspot.com/2009/11/analytical-reasoning.html&docid=vRUCjkqWJZ3RkM&imgurl=http://i46.tinypic.com/fdardu.jpg&w=448&h=336&ei=ETg6T_3yFMTirAeikPXKCA&zoom=1

the wind is blowing towards east (v_w = 120km/h),so show v_w vector towards E and the plane has to go towards north (v_p = 600km/h) then, it must head towards north-west direction with v_p such that v_p/w is towards north. So accordingly put the vectors acc to directions and then U will be able to get to the answer of your question.

Try Hard Dont Give Up :)

 the plane should fly in the northwest direction

from the right angled diagram ,,,,,,600km/h is a resultant of velocity 120km/h

600^2=120^2+v^2

v^2=345600

v=587.90km/h

and direction resultant is given by

sin(theta)=120/600

theta=sin(120/600)

theta=11.54 (degrees)

speed wrt to ground=600cos(theta) = 600cos(11.54)=600*0.51=306km/h

hope its the way it should be

correct me if im wrong

 hey brother

still wrong

we do not know the resultant velocity towards north

v^2 = 600^2 + 120^2 + 2(600)(120)cos(theta)