3. 420N
4. 11000N
The description preceding the question answers the question. Constant speed and cruising are the same, and when that is the case, that means there is no acceleration, meaning no net Force acting on the objects. That means the the frictional force in 3. must be the same as the force dragged and for 4., and the lift must equal the weight of the plane to keep it at the same altitude.
