# Three phase system represented as a vector

In multiple locations I've been seeing the following:

"Any three phase system can be defined by the vector As:

$$\ A_s = \frac{2}{3}[Ax(t) + (c)Ay(t) + (c^2)Az(t)]\$$

Where $$\ c=e^{j\frac{2\pi}{3}} \$$"

I understand this is showing the resultant of three sinusoids 120 degrees apart (from the c coefficient) but I cannot for the life of me remember where the $$\ \frac{2}{3} \$$ coefficient is coming from. Can anyone explain where this $$\ \frac{2}{3} \$$ coefficient is from? I assume its a byproduct of vector addition maybe?