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.
Coterminal Angle
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. 
Here are the steps to get the coterminal angles
Step 1 : Read the problem and Note down the given angle $\theta$.

Step 2 : Calculate the coterminal angle use the formula
Positive Coterminal angle = $\theta$ + n(360o)
Negative Coterminal angle = $\theta$ - n (360o)
Substitute the value of $\theta$ to get the desired multiples (n) of coterminal angles.

Below are given some problems based on Coterminal angles which may be helpful for you.

Solved Examples

Question 1: Find the two smallest positive angles which is coterminal with 30o.
The given angle is $\theta$ = 30o.
The two Positive coterminal angle is given by

                      $\theta$ + 360o = 30o + 360o = 390o
                      $\theta$ + 2(360o) = 30o + 720o = 750o.

Hence 390o and 750o are two angles coterminal with 30o.

Question 2: Find the two smallest negative coterminal angles for 90o.
The given angle is $\theta$ = 90o
The two Negative coterminal angle is given by

                           $\theta$ - 360o = 90o - 360o = - 270o
                           $\theta$ - 2(360o) = 90o - 720o = - 630o.

Hence -270o and -630o are two angles coterminal with 90o.