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
huh?