<7 = <5 vertical angles
<5 = <3 alternate interior angles are equal when the lines are parallel
<2 +<3 = 180 straight line ( complementary angles)
3x+20 + <3 = 180
subtract 3x+20 from each side
<3 = 180 -(3x+20)
<3 = 180 -3x-20
<3 =160 -3x
but from above
<3 also = <5 = <7 and <7 = 2x-10
160 - 3x = 2x-10
add 3x to each side
160 = 5x-10
add 10 to each side
170 = 5x
x = 34
substitute in x=34 to find angle 3
<3 = 160 -3x
<3 =160 -3(34)
<3 =58
to find angle 6
<7+<6 = 180
<7 =<3 = 58
58 + <6 = 180
subtract 58 from each side
<6 = 180 -58
<6 = 122
