[tex](s^2+5st-9t^2)-(2s^2-2st+2t^2)=\\\\=s^2+5st-9t^2-2s^2+2st-2t^2=\\\\=s^2-2s^2-9t^2-2t^2+5st+2st=\boxed{-s^2-11t^2+7st}[/tex]
Answer:
-s^2+7st-11t^2
Step-by-step explanation:
s^2 +5st - 9t^2 - ( 2s^2 -2st +2t^2)
Distribute the minus sign
s^2 +5st - 9t^2 - 2s^2 +2st -2t^2
Combine like terms
