Dear Nabhanya,

 Please consult

https://sites.math.rutgers.edu/~zeilberg/Hist21/q6S.pdf

for the full solutions.

You got 8 points (out of 10).

You started #4 correctly, and continued it the right way, EXCEPT, that you made a careless sign mistake.
So your answer was way more complicated (and wrong). I gave you 3 out 5 for doing it the right way, but accuracy is important.

 Best wishes,

   Dr. Z.