To simplify the expression $7x - 9x$, we combine the like terms:

$7x - 9x = (7 - 9)x = -2x$

So, the simplified expression is:

$-2x$

Would you like more details or have any other questions? Here are some related questions for further exploration:

1. How do you add and subtract like terms in an algebraic expression?
2. What are like terms, and how can they be identified in different expressions?
3. How do coefficients affect the simplification of algebraic expressions?
4. Can you provide an example of simplifying an expression with more variables?
5. What are the rules for simplifying expressions involving both addition and subtraction?

Tip: When simplifying algebraic expressions, always look for like terms—terms that have the same variable raised to the same power.