What does "lmk" mean ?

Grandma made an apple pie. josh and his brother joe finished 4/5 of it. then 3 friends came over and shared the left over. how much pie did each friend eat

How do you solve a question like this?

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)