Start by writing down the starting azimuth. Add 180° to obtain the back azimuth. Subtract the interior angle to obtain the azimuth of the next line. If the result is greater than 360, subtract 360.

It is the azimuth of line AB, plus 180° (to get the direction opposite to line AB, or direction BA), minus the measured angle at B: AzBC = AzAB + 180° - 70.000000° = 60.839722° + 180° - 70. ...