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The Answer is the Angle Addition Postulate
This postulate says the measure of the whole is equal to the sum of the measures of its non overlapping parts. So if you know the measurements of the right angle and the other angle, then you can subtract those from the sum of all angles to find the last.
For example, say these 3 angles added up to 180 degrees. You know there is a 90 degree angle and say the other angle is 30 degrees. We can subtract the two from the whole to find the third. 180-90-30=60. So the last angle would be the 60 degrees.
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