"OK, so like the question that i need is "The altitude "a" (in feet) of a plane "t" minutes after liftoff is given by a=3400t+600. How many minutes afterliftoff is the plane at an altitude of 21,000 feet?" Sorry, i didn't put it right earlier."
Maybe you should have put that in the question.
We want to know the number of minutes and we have the value of a, 21,000. Plug in values:
21,000 = 3,400t + 600
20,400 = 3,400t
t = 6
So, it would take \(\boxed{6}\) minutes to achieve an altitude of 21,000 feet.