“what is the particles acceleration at t=3.0 s? ”

-2 m/s²

Explanation:

Acceleration is the rate of change of velocity:

a = Δv / Δt

In other words, it's the slope of the line on a velocity-time graph.  The line at t=3 also goes through the points (2, 4) and (4, 0).

a = (4 m/s − 0 m/s) / (2 s − 4 s)

a = -2 m/s²

the answer for apex is the lowest

scientific laws are the result of testing theories and proving them to be facts.