The two angles are said to be coterminal angles if their sum is multiples of 360o sharing the same terminal side. In the figure below 150o, -210o are coterminal angles.
To calculate the positive coterminal angle and negative coterminal angle we add and subtract 360o by the given angle.
Coterminal angle Calculator is a online tool which calculates the coterminal angles instantly. Provided is the block where you are supposed to enter the angle to get the two possible and negative coterminal angles respectively.