Quick math, please help! Thank you! (v2)

How to choose the sign of the radical in the denominator of the formula for the distance from a point to a line.

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)

Find an bif a = {3, 6, 9, 12) and b = {2, 4, 6, 8, 10).