Answer
$4.5$
Work Step by Step
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.