Fill in the blank with a number to make the expression a perfect square.

To make an expression a perfect square, you need a number that, when added, results in a value that can be expressed as the square of a binomial. For example, if the expression is \(x^2 + bx\), and you want to make it a perfect square, you would complete the square by taking half of \(b\), squaring it, and then adding that number. In practical terms, if you have an expression like \(x^2 + 6x\), you would take half of 6 (which is 3), square it (which gives you 9), and thus fill in the blank with 9. This transforms the expression into \(x^2 + 6x + 9\), which factors neatly into \((x + 3)^2\). Always remember to check if the number you choose maintains the properties of perfect squares!