(2x² - 6xy + y²) + (- 6x² - 5xy - y²) + (x² + xy -7y²)
= 2x² - 6xy + y² - 6x² - 5xy - y² + x² + xy - 7y²
Now it might help to rearrange the equation a little bit to make it easier to see what to add:
= 2x²- 6x²+ x² + y² - y² - 7y² - 6xy - 5xy + xy
= - 3x² - 7y² - 10xy
This is the final simplified equation.