We have a mix of 1 inch books and 2 inch books, and the total of inches is 14, so we can write:
1x + 2y = 14
We also have a total of 9 books, so we can also write:
x + y = 9
So our system of equations is:
1x + 2y = 14
x + y = 9
Where 'x' is the number of 1 inch books, and 'y' is the number of 2 inch books.