Again, this is a triangle so the sum of the angles must be 180°, so we can say:
a+b+c=180, we are told a=129, b=2c, we will let c=M°
129+2c+c=180 combine like terms on left side
129+3c=180 subtract 129 from both sides
3c=51 divide both sides by 3
c=17°
So angle M=17°
