At

t $=10$ s$,$

a particle is moving from left to right with a speed of $7\mathrm{.}2$ m$/$s$.$ At

t $=20$ s$,$

the particle is moving right to left with a speed of $4\mathrm{.}4$ m$/$s$.$ Assuming the particle's acceleration is constant, determine the following. $($Assume the positive direction is to the right.$)($b$)$

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