You can
put this solution on YOUR website!
There is a positive and a negative value for

.
The positive solution is:

The negative solution is:

Those four angles

,

,

and

are the solution.
The reference angle is

, or

for those allergic to

.
For each (first quadrant) reference angle there is, in each of the other quadrants, a "reflection" angle that has the same absolute value for all trigonometric functions. It is

, the reflection on the y axis, for the second quadrant. It is

, the reflection on the origin, for the third quadrant, and

, or

if you want it positive, the reflection on the x axis, for the third quadrant.