Appendix F - Exercise Set - Page 1012: 27

$6$

Since the triangles are similar, corresponding sides are in proportion. This means that, $\frac{12}{4}=\frac{18}{x}$ To solve the equation for $x$, we need to cross multiply: $12\times x=4\times18$ $12x=72$ $x=\frac{72}{12}=6$ Therefore, the missing length is $6$ units.