Answer
$6$ minutes
Work Step by Step
The altitude is $21,000$ feet so $a=21,000$.
Substitute $21.000$ to $a$ in the given equation to obtain:
$$\begin{align*}
a&=3400t+600\\
21000&=3400t+600\\
21000-600&=3400t &\text{Subtract 600 from each side.}\\
20400&-3400t &\text{Simplify.}\\
\frac{20400}{3400}&=\frac{3400t}{3400} &\text{Divide 3400 to both sides.}\\
6&=t
\end{align*}$$
Thus, the altitude if the plane will be at $21,000$ feet after $6$ minutes.
