Hi there! Thank you for the opportunity to help you with your question!
When we have sums such as
3x^2y - 9x
We can always look for things that all terms have in common. The first thing to notice, is that both terms are divisable by 3. Therefore, I can "factor out" a three from both terms. This is the "inverse" of the distributive property:
[If you multiplied the 3 back in, you would go back to the original expression.]
Now, note that the two terms still have something in common, an "x". I can similarly factor it out:
3*x*(xy - 3)
And this is the factored out expression.
Please let me know if you need any clarification. Always glad to help!