The altitude "a" (in feet) of a plane "t" minutes after liftoff is given by a -3400t+600. How many minutes after liftoff is the plane at an altitude of 21,000 feet?

21000 = 3400 t + 600 -> 21000 - 600 = 3400 t --> 20400 = 3400 t --> 20400/3400 = t --> t = 6  min