Answer:
[tex]\mathrm{D.\:}\sqrt{126}[/tex]
Step-by-step explanation:
All three triangles in the figure are similar. Therefore, we can set up the following proportion:
[tex]\frac{18}{m}=\frac{m}{7}[/tex]
Cross-multiply to solve:
[tex]m^2=18\cdot 7 = 126, \\\\m=\fbox{$\sqrt{126}$}[/tex].
