I total of the outside of a triangle is 360 and so the outside of angle 51 will be 129 because the pairs will equal 180 so you subtract 180 to 51 to get 129. 112 and C will need to add up to 180 and so will A and B. So since you now have two of the outside angles and you know they need to add up to 360, you take both angles and subtract it to 360 to 119.
