Two angles are coterminal if they end up in the same ray if they are given in standard position (they both begin in the positive x-axis).
To find out if two angles are coterminal we can add or substract a whole revolution (2pi) or more revolutions to one of them and we need to get the other one.
In this case we will need that the equation:

has an integer solution for n. This will mean that we can go from one angle to the other with a given number of revolutions.
Solving for n we have:

This means that if we begin in the angle -pi/6 and give two whole revolutions we end up in the angle 46/12pi.
Therefore, the angles are coterminal.