The plane will be at an altitude of 21,000 feet, 6 minutes after lift off.
For any equation, x = y + a, if we are given the value of any one of the variables, that is either x or y, we can find the value of the other.
Here, we are given an equation
a = 3400t + 600
where a = altitude of the plane in feet
and t = time (in minutes) passed after the plane's lift off
If the altitude of the plane is 21,000 feet
⇒ a = 21,000
Substituting this in the equation, we get
21,000 = 3400t + 600
21,000 - 600 = 3400t
20,400 = 3400t
3400t = 20,400
t = 20,400/ 3,400
t = 204/ 34
t = 6
Thus, the plane will be at an altitude of 21,000 feet, 6 minutes after lift off.
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