Answer:
72 screws are required to make 6 chairs.
108 screws are required to make 9 chairs.
Explanation:
We are given the following in the question:
Gary needs 24 screws for every 2 chairs he builds.
We calculate the equivalent ratio to calculate number of screws required.
Ration of chairs to screws can be calculated as:

This can be interpreted hat 1 chair needs 12 screw.
We have to find number of screws required to make:
1. 6 chairs
Let x be the number of screws required to make 6 chairs, the we can write,

72 screws are required to make 6 chairs.
2. 9 chairs
Let y be the number of screws required to make 9 chairs, the we can write,

108 screws are required to make 9 chairs.