The interior angles of a 7 sided figure add up to 900o That's because if you pick any vertex of an angle and draw diameters so that you create triangles, you will get 5 triangles. In general the interior angles will add up to (n - 2)*180 where n is the number of sides of a polygon.
So the simple way to do this is to equate the seven angles + x to 900
Equation
x + 125 + 126 + 158 + 120 + 139 + 121 = 900
Solve
x + 789 = 900 Subtract 789 from both sides.
x = 900 - 789
x = 111 <<<<< answer
