All that data about the jet was to throw you off. If the thing has zero acceleration (implied by "cruising along") then of course the vector sum of all forces on it must be zero. The thrust of the engine, the lift on the wings, the air drag force, and the force of gravity all together have a vector sum of zero. If they didn't, the plane would accelerate.
If you do not get this, you need to meditate about it awhile. Think about other cases of constant velocity (cars, runners...) and of course it will seem as though the sum of forces is not zero. Keep at it until the zero total seems OK to you. You may have to talk it over with others. If you are not willing to do this, you are doomed, and you need to be cursed out. Now beam me back, Scottie, back to the main phys page so i can get in trouble all over again. Or maybe you came from index, where there is a lot of stuff.