In this expression, Marianne claims that 2x and - x are not like terms because - x does not have a coefficient. Is she correct? Explain your reasoning.

No, Marianne is not correct. The coefficient of - x is - 1. The "1' is just omitted and only the negative is kept because showing 1 is unnecessary as 1 multiplied by anything is itself.

Sample Response: Marianne is incorrect because like terms are terms that have the same variables, raised to the same powers. The terms - x and 2x fit the definition for like terms. The term - x has a coefficient of - 1.