# How do you determine the missing coordinate A (0, d), B(d, 0) if slope is undefined?

$d = 0$ results in undefined slope. If $d \ne 0$ then slope is defined and is $- 1$
If $d = 0$ then both points are $\left(0 , 0\right)$ so the slope is indeterminate, so undefined.
If $d \ne 0$ then slope $m$ is defined by the formula:
$m = \frac{\Delta y}{\Delta x} = \frac{0 - d}{d - 0} = - 1$