Note : All the BOLD letters are vectors.
As we have to go north, we'd like to apply all our strength towards N.
i is a unit vector towards E (from W) and j is unit vector towards N.
Let speed of plane w.r.t. ground be u
Speed of wind ( say w) = 120 i.
Speed of Plane w.r.t. air (say s) = u - w = 600 j.
Then, u = s + w
u = 600 j + 120 i.............!!!!!
ans!!!