Appendix F - Exercise Set - Page 1013: 30

$48$

Since the triangles are similar, corresponding sides are in proportion. This means that, $\frac{2}{24}=\frac{4}{x}$ To solve the equation for $x$, we need to cross multiply: $2\times x=24\times4$ $2x=96$ $x=\frac{96}{2}=48$ Therefore, the missing length is $48$ units.