360 +360 - 180 =540

Because the sum of the four angles of an arbitrary quadrilateral is 360, and the straight angle is180.

So:

360- (Angle 3+ Angle 4+ Angle 8)+360- (Angle 5+ Angle 6+ Angle 7) = 180.

And angle 8 = angle 1+ angle 2.

To sum it up

The sum of angles 1, 2, 3, 4, 5, 6 and 7 is:

360 +360 - 180 =540