I don't understand this math please anyone help

If you were to rotate the flag to the right you will see that the inside seams of the shadow box are the same length as the edges of the folded flag. Meaning that if we were to just consider the length measurements of the folded flag then the flag would fit. However, we are also given the degree of which the point of the flag is folded, so we need to make sure that matches with the degree angle of the shadow box. The angles are the same therefore the folded flag should fit perfectly into the shadow box.