The flat line is always 180* the angle on the right says 138*. So you subtract 138 from 180 and you get 42*. So you put 42* as the Acute angle next to 138* on C.
It’s been a while since I did this next part so I may be wrong. But once you have the angle of two sides you add them and subtract them from 180 to get the third angle of the rectangle. So 82+42= 124, 180-124=56. So 56 is A