9. (5 points) If 2 harvesters can harvest 450 acres of wheat in 3 days, then using 7 harvesters to harvest 2100 acres of wheat will take days.

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【Analysis】First, find the work efficiency of each machine per day, then find the work efficiency of 7 machines in one day. Since work volume $\div$ work efficiency $=$ work time, solve accordingly.

【Solution】Solution: $2100 \div(450 \div 3 \div 2 \times 7)$
$$
\begin{array}{l}
=2100 \div(75 \times 7) \\
=2100 \div 525 \\
=4 \text { (days), }
\end{array}
$$

Answer: It takes 4 days to harvest 2100 acres of wheat using 7 harvesters.
Therefore, the answer is: 4.