A straight line is 180°, so you could do:
m∠1 + m∠2 = 180° And you can plug in the angle you have into the equation.
m∠1 + 57° = 180° To get the missing measure, you subtract 57° on both sides of the equation to get m∠1 by itself.
m∠1 = 123° And this is your answer.