1) A heptagon has (7-2)*180 = 900 degrees totalling its internal angles.

All of the internal angles in this heptagon total 8x. Therefore, x = 900/8 = 125°

2) I won't bother explaining because you've done it and the explanation is lengthy, but I got it to be a hexagon. Am I right?

3) The external angles of a polygon always add to 360°. The average external angle is (8+12)/2 = 10°, so there are 360/10 = 36 sides.