The position $x,$ in meters, of an object moving along the $x-$axis is given as a function of time $t,$ in seconds, as

$x(t)=0\mathrm{.}0807t^4-0\mathrm{.}129t^3+0\mathrm{.}749t^2+2\mathrm{.}97t-8\mathrm{.}99$

Find the object's acceleration at time $t=3\mathrm{.}89s.$

acceleration: