Sorry for all the math questions but i do not understand this at all. someone explain and write the equation for me ? ? they are the exact doesn't make sense to me:

the sum of three times a number (z) and 15 is equal to 21.

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)

Match the following reasons with the statements given to create the proof. 1. do = ob, ao = oc sas 2. doc = aob given 3. triangle cod congruent to triangle aob vertical angles are equal. 4. 1 = 2, ab = dc if two sides = and ||, then a parallelogram. 5. ab||dc if alternate interior angles =, then lines parallel. 6. abcd is a parallelogram cpcte

Acube has a volume of 800 cubic inches. whatis the length of an edge of the cube, in inches? a. 23/100b. 43/50c. 83/100d. 1003/2e. 1003/8