Answer:
Assuming they're unit vectors (length of 1)
You simply add them together.
The vector pointing right can be described with X and Y
(1, 0).
The vector pointing down
(0, -1).
The numbers are taken from the axis in X and Y directions.
If you add them together you get a final vector.
(1, - 1)
This gives you -45° pointing South East.
Or 360°-45°=315°
