Which of the following expressions can be combined together?

• A. 3xy and 4xy
• B. 2x² and 3y²
• C. -5a and b
• D. 7y and 2x

Answer: (A)

The expression that can be combined together is 3xy and 4xy because they are like terms. Like terms are terms that have the same variable portions raised to the same powers. In this case, both terms contain the variable 'xy' with the same power, which allows you to combine the coefficients (3 and 4). When you add them, you get (3 + 4)xy, which simplifies to 7xy.

Each of the other expressions consists of terms that are not like terms, which prevents them from being combined. For instance, 2x² and 3y² have different variables and cannot be combined through addition or subtraction. Similarly, -5a and b involve different variable representations, and 7y and 2x do not share the same variable components either. Therefore, only the first pair can be successfully simplified together.