since the figures are similar, then we can just use proportions on the corresponding sides.
[tex]\bf \cfrac{\textit{large figure}}{\textit{small figure}}\qquad \qquad \cfrac{x}{6}=\cfrac{21}{9}\implies \cfrac{x}{6}=\cfrac{7}{3}\implies 3x=42 \\\\\\ x=\cfrac{42}{3}\implies x=14[/tex]
