Guys me im so stuck been on this question for 1 week

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)

The difference of three and the reciprocal of a number is equal to the quotient of 20 and the number. find the number.

Heather is riding a quadratic function that represents a parabola that touches but does not cross the x-axis at x=-6