2.在建築物，電梯上升9樓地面。然後下降11層。之後，它上升6層。在哪層電梯才停下來？
↑這條數該怎列式??
更新1:
A plane is flying at an altitude of 9000m. Due to the rough weather condition, the plane makes the following changes in altitude: -480m , -210m, +350 m, +310 m , -490 m (a) Find the total change in altitude of the plane. (b) Find the final altitude of the plane after the changes.