First calculate distance traveled after 6 min.
400 * (6/60) = 400*(1/10) = 40 km
Next, draw triangle with 2 sides being 45 and 40. The included angle between these sides is 15, the angle of elevation. The third side is distance from plane to storm cloud.
Next, use Law of cosines to find distance.
d = sqrt(40^2 + 45^2 - 2(40)(45)cos(15))
d = 12.152 km