1. The problem is to understand the number 9. 2. Number 9 is a single-digit integer and is the square of 3 since $9 = 3^2$. 3. It is an odd number and is the highest single-digit number in the decimal system. 4. In algebra, 9 can be used as a constant or coefficient in equations. 5. For example, if we have the equation $x + 9 = 15$, we can solve for $x$ by subtracting 9 from both sides: $$x = 15 - 9$$ $$x = 6$$ 6. So, 9 is a number that can be used in various mathematical contexts and operations.