Answer
$\frac{j}{d} = \frac{d}{g}$
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 $j$ is the hypotenuse of the large triangle. Additionally, $g$ is the shorter side of one of the triangles formed by the altitude of the large triangle.
What we want is the hypotenuse of this smaller triangle, which is represented by $d$, and the corresponding shorter leg of the large triangle, which is also represented by $d$.
The proportion should read:
$\frac{j}{d} = \frac{d}{g}$
