Can someone please help me out with this. Pls don't just say A, B, C, or D, like i need explanation pls. I am begging you, I cannot fail this

Idon’t understand this.. will someone ? will mark brainliest along with 20 points.

The position of a moving particle is given by the position function: f(t)=-9t-t^2-0.2t^3+0.1t^4 0 a. at what time does the particle reverse direction? b. when is the displacement positive? (round one decimal place and answer in interval notation) c. when is the displacement negative? (round one decimal place and answer in interval notation) d. when is the particle’s acceleration positive? (round one decimal place and answer in interval notation) e. when is the particle’s acceleration negative? (round one decimal place and answer in interval notation)

Including 6% sales tax, an inn charges $135.68 per night. find the inns nightly cost