Which expression represents the addition of a variable and a constant?

The expression that represents the addition of a variable and a constant is x + 3. In this expression, "x" serves as the variable, which can take on different values, and "3" is the constant, which always stays the same. This expression explicitly shows the operation of addition, indicating that 3 is added to the value of the variable x.

In contrast, the other options do not represent the addition of a variable and a constant. For instance, x - 3 indicates subtraction rather than addition, 3 - x also reflects a subtraction operation, and 3x denotes multiplication between the constant 3 and the variable x. Understanding these distinctions is crucial for recognizing how different operations affect the relationships between variables and constants in mathematical expressions.