Combining like terms by adding and subtracting requires the terms to have the same base and the same:

1. Like terms must match in more than one way. 2. Simplifying expressions often means combining like terms. 3. When you combine like terms by adding and subtracting, you need the same base. 4. You also need the same exponent. _Source: OpenStax Elementary Algebra 2e (CC BY 4.0), Ch 6 'Polynomials', sections 6.1-6.8_