Statement Reason
m∠EGF = 60° & m∠AGF = 90° Given
∠EGF + ∠AGF + ∠AGB = 180° Definition of supplementary angles
60° + 90° + ∠AGB = 180° Substitution
150° + ∠AGB = 180° Add the the similar values
∠AGB = 180° - 150° Subtraction property of equality
∠AGB = 30° Answer
You are free to arrange any of the reasons (and statements) as you wish.
I just want to give you the main idea.
Hope this helps.