The given expression is:

$4x + 5x$

To simplify this, you combine like terms. Both terms have the same variable $x$, so you can add the coefficients (4 and 5) together:

$4x + 5x = (4 + 5)x = 9x$

Thus, the simplified expression is:

$9x$

Let me know if you'd like further clarification or have more questions!

Here are some follow-up questions:

1. How do you simplify expressions with different variables?
2. What happens if you have more terms with different exponents of the same variable?
3. How do you factor out a common term from an expression?
4. What is the difference between like terms and unlike terms in algebra?
5. How do you distribute terms in an algebraic expression?

Tip: Always combine like terms first to simplify expressions more efficiently!