Alternatively, you know:
(5, -3) = r*(cos(theta), sin(theta)) where theta = 90 - bearing
- think 5 - 3i = r cis(theta)
also, r = length = sqrt(34)

so, really:
(5, -3) = (r sin(bearing), r cos(bearing))

=> tan(bearing) = 5 / -3
now, there are two options, one of which is obviously true, one of which is 180 degrees out.


(BTW, if it's in C++, you can use bearing = atan2(5, -3))