Confused about question

Question

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 21000 feet?

Answer 1

We are going to solve for t if a is 21,000 (ft).

 

21,000=3400t + 600

-600                - 600

20,400=3400t

Divide each side by 3400

6=t (minutes)

Answer 2

Set a(t) = 21000 ft, and solve for t.

 

21000 = 3400t + 600

 

Solve for t.  Can you finish from here?