Appendix F - Exercise Set - Page 1013: 29

$4.5$

Since the triangles are similar, corresponding sides are in proportion. This means that, $\frac{6}{9}=\frac{3}{x}$ To solve the equation for $x$, we need to cross multiply: $6\times x=3\times9$ $6x=27$ $x=\frac{27}{6}=4.5$ Therefore, the missing length is $4.5$ units.