#### Answer

$\frac{n}{a} = \frac{a}{c}$

#### 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 $a$ is the hypotenuse of that same triangle. Additionally, $a$ is the longer leg of the large triangle. What we want is the corresponding hypotenuse of the large triangle, which is represented by $c$.
The proportion should read:
$\frac{n}{a} = \frac{a}{c}$