the inside of quadrilateral is always 360°.So add 75°,90° and 75° and then subyaract from360°.
x+75°+75°+90°=360
×+240°=360°
x=360°-240°
x=120°
Answer:
(_1,_7),(_2,_6),(_3,_5),(_4,_4)(_5,_3)