Answer:
40°
Explanation:
The reference angle is the positive acute angle created by the terminal arm and the x-axis.
The highlighted red in the picture below shows what we're looking for.
The arm rotated 220° (but 'backwards' so the value given is negative).
|-220°| - 2(90°) <= Subtract two right angles for two quadrants
= 220° - 2(90°)
= 220° - 180°
= 40°
Therefore, the reference angle is 40°.
If you got 50°, you probably calculated the angle with the terminal arm and the y-axis. Remember to always use the nearest side of the x-axis!