Which is an equation of a direct proportion?

Jayne is studying urban planning and finds that her town is decreasing in population by 3% each year the population of her town is changing by a constant rate

Through non-collinear points there is exactly one plane.

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)