Explanation:
in a parallelogram we have 2 equally large angles and 2 equally small angles.
the sum of all 4 angles must be 360°.
so let's first determine what angles are together 204°.
if it would be 2 neighboring angles, it would be the sum of a large and a small angle.
and the sum of the remaining 2 angles would be the same (again 204°).
but that that would together be 408°. too much.
so, it can only be that the 2 given angles are a pair of equal angles. and each would be
204/2 = 102°
as they are larger than 90°, they are the large angles in the parallelogram.
so, the other 2 angles are the pair of equally large small angles. together they have
360 - 204 = 156°
and each one is then
156/2 = 78°