Answer
$\frac{n}{h} = \frac{h}{m}$
Work Step by Step
All three triangles are similar to one another, meaning that the sides of the triangles are proportional.
We can see that $n$ is the longer leg of the larger triangle formed by the altitude of the large triangle, and $h$ is the shorter leg of that same triangle. Additionally, $h$ is the longer leg of the smaller triangle formed by the altitude of the large triangle. What we want is the corresponding shorter leg in this smaller triangle, which is represented by $m$.
The proportion should read:
$\frac{n}{h} = \frac{h}{m}$
