3. x+100 = 90 alternate interior angles are equal
subtract 100 from each side
x+100 - 100 = 90-100
x = -10
4. 17x+1 = 120 alternate exterior angles are equal
subtract 1 from each side
17x+ 1 -1 = 120 -1
17x = 119
divide each side by 17
17x/17 = 119
x =7