t

t $=10$ s$,$

a particle is moving from left to right with a speed of $7\mathrm{.}8$ m$/$s$.$ At

t $=20$ s$,$

the particle is moving right to left with a speed of $2\mathrm{.}2$ m$/$s$.$ Assuming the particle's acceleration is constant, determine the following. $($Assume the positive direction is to the right.$)$

$($a$)$

its acceleration $($Enter your answer in m$/$s$2.$ Indicate the direction with the sign of your answer.$)$

m$/$s$2$

$($b$)$

its initial velocity $($Enter your answer in m$/$s$.$ Indicate the direction with the sign of your answer.$)$

m$/$s

$($c$)$

the instant $($in s$)$ when its velocity is zero

s