Which is best estimate of each percent? 21/ of 387?

How do i make someone brainliest? (if you answer me, i might be able to make you brainliest! )

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)

How do i simply this expression (quadratic formula basis) on a ti-84 or normal calculator?