Can someone please me i don't understand

The perimeter of a movie screen is 54 meters. the screen is 18 meters wide. how tall is it?

Find the greatest common factor of the followig monomials 21u^5v,3u^4v^6, and 3u^2v^5

The position of a moving particle is given by the position function: f(t)=-9t-t^2-0.2t^3+0.1t^4 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)