How to simplify 3x^2y - 5xy^2 + 6x^2y?

First you need to get your presentation straight. We use parentheses to group things.

(3x^2)y - 5xy^2 + (6x^2)y
3x^(2y) - 5xy^2 + 6x^(2y) No way to tell which was intended.

Second, you need to learn what the words mean. That expression, however it is interpreted, is already as simple as it gets. You might have meant to evaluate it, but that requires an equal sign.