At t$1$

$=6\mathrm{.}0$ s

an object's velocity is v$1$

$=10\mathrm{.}190$ m$/$s

to the EAST, and at t$2$

$=12\mathrm{.}0$ s

its velocity is v$2$

$=18\mathrm{.}650$ m$/$s

to the WEST. Assuming a constant acceleration, Take EAST as the positive direction. What is its acceleration with a proper sign? Keep $3$ digits after the decimal point.