The expression given is:

$3y + 3x + 6x$

To simplify, first combine like terms. The terms $3x$ and $6x$ are like terms because they both involve $x$. Adding them together:

$3x + 6x = 9x$

So, the simplified expression becomes:

$3y + 9x$

Would you like more details or have any questions on this?

Here are five related questions to expand on this:

1. What are like terms in algebraic expressions?
2. How can you factor the expression $3y + 9x$?
3. Can you apply the distributive property to simplify this further?
4. What is the difference between simplifying and solving an algebraic expression?
5. How do you combine terms with different variables?

Tip: Always remember to combine like terms when simplifying algebraic expressions. Terms are "like" if they contain the same variable raised to the same power.