angle EOD is congruent to angle COF straight angles theorum
measure angle EOD is equal to measure angle COF definition of congruence
measure angle EOD equals 3x given with model
3x equals measure angle COF substitution property of equality
measure angle COF equals 3x symmetric property of equality
measure angle AOB equals 180, definition of a straight angle
measure angle AOC plus measure angle COF plus measure angle FOB equals measure angle AOB angle addition postulate
measure angle AOC equals 2x given with model
measure angle FOB equals x given with model
2x +3x +x =180 substitution property of equality
6x =180 combine like terms
x =30 division property of equality
