Which algebraic concept is assessed when students solve the equation 12 + 5 = 9 + ?

The correct answer focuses on the concept of equivalency of numbers, which is fundamental in algebra. When students are asked to solve the equation 12 + 5 = 9 + ?, they must recognize that both sides of the equation must represent the same value for the equation to hold true. This leads them to find the missing number that balances the equation.

In this scenario, students need to understand that the left side presents the total sum of 12 + 5, which equals 17. The right side, 9 + ?, requires them to identify what must be added to 9 to also yield 17. Thus, the student will determine that the missing number must be 8 (since 9 + 8 = 17).

This process of finding a value that makes both sides equal reinforces the important algebraic concept of equivalency, which is essential for solving equations. It helps students build a foundational understanding of how numerical relationships work, which is crucial in algebra and further mathematical studies.

Other options such as patterning, variables, and graphing do not specifically align with the primary focus of this question, which is testing the students' understanding of equal values in the context of an equation.