physics
posted by Donna .
Several people are riding in a hotair balloon. The combined mass of the people and balloon is 284 kg. The balloon is motionless in the air, because the downwardacting weight of the people and balloon is balanced by an upwardacting 'buoyant' force. If the buoyant force remains constant, how much mass should be dropped overboard so the balloon acquires an upward acceleration of 0.28 m/s2?

weight=ma
but m is the weight left
weight=(284Weight)*.28m/s^2
solve for weight 
but if you have two variables how do you solve? I understand that force=ma so the normal force is (284)(9.8)=2783.2, but I don't really understand how to determine when acceleration is .28