A particle is moving in one dimension (along $x$ axis) under the action of a variable force. It's initial position was $16 \mathrm{m}$ right of origin. The variation of its position $\left(x\right)$ with time $\left(t\right)$ is given as $x=–3t^3+18t^2+16t,$ where $x$ is in $\mathrm{m}$ and $t$ is in $\mathrm{s}.$ The velocity of the particle when its acceleration becomes zero is _________ $\mathrm{m} {\mathrm{s}}^{-1}.$