-120 degrees equals 240 degrees by unit circle. Also we can find it by knowing that a circle is 360° and just adding -120° + 360° and that equals 240 degrees.
ALSO, So, -120° is -2pi/3 radians, so we can do the same thing, but just in a relation to radians, so we know 360° equals 2pi, but if we don't we can find it
The goal is to find the legs of the right triangle, so you can find the hypotenuse. The hypotenuse is the distance between the two points. See the attached image.