A math expression is different from a math equation. An equation will always use an equivalent operator between two math expressions. In an expression only like terms can be combined.
These like terms can be combined through addition or subtraction.
For instance you know that you can add 2 3 and get 5. Like terms are terms that have the same variables and exponents. In early grades children are expected to write math expressions using numbers and. Like terms are two things that can added.