x + 123 = 180 (angle on a straight line)
subtract 123 from both-side of the equation
x + 123 - 123 = 180 -123
x=57
x + y = 180 (angle on a straight line)
57 + y = 180
subtract 57 from both-side of the equation
57 - 57 + y = 180 -57
y = 123
