You measure it very, very carefully!
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the angle on a straight line is 180 degrees, the angle of semicircle at centre is 180 degrees
Subtract the angle from 90 degrees and you have the complement Subtract the angle from 180 degrees and you have the supplement
56 degrees
To find an acute angle you need to find out if it is less than 90 degrees.
You cannot because there is not enough information.