Please use FileSystem tools to finish the following task:

### Task Description

Your task is to find all files that contain a substring of 30 or more characters that also appears in `large_file.txt`. **You are not allowed to use python code.**

### Task Objectives

1. **Read the reference file** `large_file.txt` to understand its content
2. **Examine each file** from file_01.txt to file_20.txt
3. **Find files** that contain a substring of 30 or more characters that matches a substring in `large_file.txt`
4. **Create a file `answer.txt`** and write the results to it with the following format:
   - One line per matching file
   - Format: `filename.txt,start_position`
   - Where start_position is the character position (1-indexed) of the matching substring in `large_file.txt`
   - Do not add any things else other than `filename.txt,start_position`.
