9. (15 points) There is a batch of homework. Teacher Wang originally planned to grade 6 books per hour. After grading for 2 hours, he decided to grade 8 books per hour, and as a result, he finished 3 hours ahead of schedule. How many books are in this batch of homework?


【Analysis】According to the problem, the total number of assignments remains unchanged, meaning the total amount of work is constant. Therefore, the planned and actual work efficiency is inversely proportional to the working time. Based on this, let's assume it was originally planned to take $x$ hours to complete the grading, and set up an equation to first find the number of hours originally planned, then use work efficiency $\times$ working time $=$ work volume to solve the problem.

【Solution】Solution: Let the original plan be to complete the grading in $x$ hours. According to the problem, we have:
$$
\begin{array}{c}
6 \times 2+8(x-3-2)=6 x \\
12+8 x-40=6 x \\
8 x-6 x=28 \\
2 x=28 \\
x=14 . \\
6 \times 14=84 \text { (books); }
\end{array}
$$

Answer: There are 84 books in this batch of assignments.