Answer:
129
Explanation:
Every angle would be similar to each other with having only TWO different degrees...
Each line ( <----------------------> ) equals 180 (never 360, b/c that would be a circle)
Now with you only having 51 as a degree, you need to do 180-51 which gives you 129, with that given, this tells you number 1 and 3 is both 129, and angle 2 is 51.