1. Fold the triangular paper piece $A B C$ along a line parallel to $B C$, the folded part intersects $B C$ at points $F, G$. Given $B C=9 \mathrm{~cm}, F G=3 \mathrm{~cm}$, then the length of the crease $D E$ is $\qquad$ $\mathrm{cm}$.
$6$