Answer:
1. Because every angle on the opposite of 6 and 7 are corresponding to those 2 angles, meaning they are going to be the same value.
2. The sum of the listed angles 6, 7, 10 and 11 is 360 degrees because they are all right angles.
3. In a parallelogram the the opposite angles are congruent, and the sum of the interior are all an equal number, adding up to 360, each being 90 degrees on its own.