This is an exterior angle because x and y=180.
now let us pretend those sides are z. Each side is equal.
So x=z
so z=y
now 180/3 is 60
so x or z is 60.
now we use exterior angle and now we do 180-60 is 120.
SO
X=60
Y=120
now we make sure by z times 2 should equal y and z=x and x=60 and 60 time 2 is 120 so it is correct.