Note that a triangle = 180 degrees, and that a square inside the triangle located in a corner means a right angle or 90 degrees.
The sum of the exterior angle is the addition of the two isolated interior angles:
90 + 53 = u
u = 143
The sum of a straight line is 180 degrees. Subtract 180 with 143
180 - 143 = 37
The measure of the missing interior angle is 37 degrees
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