(-4w² - 9n) - (-15w²)
Treat the minus sign in the middle as if it's -1 and distribute.
(-4w² - 9n) -1(-15w²) = (-4w² - 9n) +15w²
Take out parentheses -4w² - 9n +15w²
Combine like terms. Two of these terms have a w² so we put them together
11w² - 9n
That's as simplified as it could get