Answer:
Total 14 children were still in the library
Explanation:
The total number of students in a library is 21. After that 7 children left the library.
If x children arrive in the library, the number of students in the library are increased by x. It means we have to add x units to the total number of students in the library.
If y children left the library, the number of students in the library are decreased by y. It means we have to subtract x units from the total number of students in the library.
Since the 7 children left the library, therefore the number of students reduced by 7.


Therefore 14 children were still in the library.