You should not play with weight. L = W is valid in level flight at any altitude. What you have to do is, keep weight constant (unless you want to account for consumed fuel weight in going from sea-level to 11km), and take the correct density of air at that altitude and then find the velocity that would be required to maintain the level flight condition. Change should be found in velocity and thus in thrust required and not in the weight.
Dr. Nandan Kumar Sinha Date: