Let's combine the like terms from the expression:

$-y^2 - x - 3y^2 - xy$

Step-by-step breakdown:

1. Combine the $y^2$ terms: $-y^2 - 3y^2 = -4y^2$

2. The $x$ and $xy$ terms are not like terms with $y^2$, so they remain as they are: $-x \quad \text{and} \quad -xy$

Thus, the expression simplifies to:

$-4y^2 - x - xy$

This is the final simplified expression.

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

Here are 5 related questions for you:

1. What are "like terms" in algebra?
2. How do you combine terms with different variables?
3. What is the significance of the distributive property when combining terms?
4. How can we factor expressions like $-4y^2 - x - xy$?
5. How does this process apply to solving equations?

Tip: Always look for terms that have the same variable raised to the same power. These are the only ones you can combine!