okay so since you're just adding these two polynomials together and there's no special operations going on, you can just ditch the parenthesis and start simplifying.
4s² + 2st + t - 3s² + 5st - 4t
4s² - 3s² + 2st + 5st + t - 4t
s² + 7st - 3t
that should be your answer