The expression "3x + 5x" involves combining like terms, which are terms that have the same variable raised to the same power. In this case, both terms contain the variable x. To simplify the expression, you add the coefficients (the numbers in front of the variable) together. When you add 3 and 5, you get 8. Therefore, "3x + 5x" simplifies to "8x." This means that the total value of the expression is represented by 8 times the variable x. Simplifying expressions like this is a fundamental skill in algebra.