ok this is simple. first, lets look at the 123 angle, so the entire line is 180. subtract 123 from 180 and you have 57. a triangle will always add up to 180 degress aswell. so add 58 and 57 and thats 115. so since a triangle will always add up to 180, subtract 115 from 180 and you have the missing angle:
65!
