As we know, the exterior angle plus the interior angle adds up to 180°, so the missing angles moving clockwise from the top are: 180 – 71.4 = 108.6° 180 – 56.4 = 123.6° ...

Interior angles in a quadrilateral add up to 360°. Symmetry and 2D shapes. Parts of a shape on either side of a line of symmetry will be exact mirror images of each other.