The problem you have shared is related to completing a magic square, where the numbers 1 to 9 are used exactly once, and the sum of the numbers in each row, column, and diagonal must equal 15.

Given partial 3x3 magic square:

8 ( ) 
7 (a) 
9

The problem is asking for the value of $a$ in this grid.

To solve:

1. In a 3x3 magic square, the sum of every row, column, and diagonal must be 15.
2. The middle cell (which is $a$ here) is crucial, as it interacts with multiple rows, columns, and diagonals.
3. We already know that in a typical 3x3 magic square with numbers 1 to 9, the center value is always 5. This value balances all the sums perfectly.

So, $a = 5$.

Would you like a detailed explanation of how a magic square is constructed or how we can verify the solution?

Related questions:

1. How do we calculate the sum of rows, columns, and diagonals in a magic square?
2. Can the magic square be constructed in multiple ways with the same sum?
3. Why is the number in the center of a 3x3 magic square always 5?
4. How does the magic square relate to other number patterns?
5. What are the other properties of magic squares?

Tip:

In any 3x3 magic square with numbers 1 to 9, the middle number will always be 5 to balance the sums of rows, columns, and diagonals.