Solution: If the library has X books on the first floor and (173-X) books on the second floor.

2(X-38)= 173-X can be obtained from the meaning of the question.

2X-76= 173-X

2X+X= 173+76

3X=249

The solution is X=83.

So the library has 83 books on the first floor and 90 books on the second floor.

I wish you progress in your study ~