Final answer:
The degree of the expression 3x² is 2, as this indicates the highest power of the variable x within the expression.
Step-by-step explanation:
The degree of an expression in algebra represents the highest power of its variables. In the expression 3x², we have the variable x raised to the second power. Therefore, the degree of the expression 3x² is 2. It is important to note that cubing of exponentials is a different concept where, for example, if we cube x², we get x6 (as we multiply the exponent by 3), which then would have a degree of 6. However, just the expression 3x² itself refers to a second-degree polynomial due to the exponent on x.