Hi,
in the following image we want to find the sum of following angles:A+B+C+D+E+F+G
View attachment 1776
I have numbered the angles in that seven sided convex polygon.The sum of their measures is \(\displaystyle \sum\limits_{k = 1}^7 {m(\angle k) = 900^o } \).Using the external angle theorem we get:\(\displaystyle m(\angle 1) = m(\angle B) + \left( {180^o - m(\angle 2)} \right)\) \(\displaystyle m(\angle 2) = m(\angle C) + \left( {180^o - m(\angle 3)} \right)\)\(\displaystyle m(\angle 3) = m(\angle D) + \left( {180^o - m(\angle 4)} \right)\)\(\displaystyle m(\angle 4) = m(\angle E) + \left( {180^o - m(\angle 5)} \right)\)\(\displaystyle m(\angle 5) = m(\angle F) + \left( {180^o - m(\angle 6)} \right)\)\(\displaystyle m(\angle 6) = m(\angle G) + \left( {180^o - m(\angle 7)} \right)\)\(\displaystyle m(\angle 7) = m(\angle A) + \left( {180^o - m(\angle 1)} \right)\)Now if we add these seven equations we see your professor's answer is correct.