[填空题]
The diagram below shows a sector of a ci2.7q ex him i:3dy* fc,nnbn.wrclt3msdgd v7h( *zt.e(pe with radius r where $\mathrm{AO} \hat{\mathrm{B}}=x $ radians and the length of the $\operatorname{arc}$ A B is $\frac{4}{x} \mathrm{~cm}$ .
$\text { Given that the area of the sector is } 27 \mathrm{~cm}^{2} \text {, find the length of the arc } A B \text {. }$
