Have a look inside the back of your sofa. Or can you remember where you used it last?
Alternatively, you could be more helpful in asking your question. Missing angle from what? What information is available?
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The answer depends on which variable is missing.
butty mutt
180 minus two known angles = unknown angle
The total of the three interior angles in a triangle is 180 degrees. if you know two of the angles, subtract them from 180 and that gives you the third angle.
That will depend on the shape but if it's a triangle then the missing angle is 54 because there are 180 degrees in a triangle.