Appendix F - Exercise Set - Page 1012: 28

$8$

Since the triangles are similar, corresponding sides are in proportion. This means that, $\frac{7}{14}=\frac{4}{x}$ To solve the equation for $x$, we need to cross multiply: $7\times x=4\times14$ $7x=56$ $x=\frac{56}{7}=8$ Since the two unknown sides are equal, both have a length of $8$ units.