Explanation

6x + 9 is an expression

So, the expression 6x + 9 represents the sum of 6 times the value of x and 9.

The value of x can vary, but the expression remains the same structure.

For example, if x = 2, then the expression becomes:

6(2) + 9 = 12 + 9 = 21

If x = -1, then the expression becomes:

6(-1) + 9 = -6 + 9 = 3

Therefore, the expression 6x + 9 is a mathematical representation that combines a variable (x) with constants (6 and 9) using addition.