I'm trying to figure out the necessary azimuth and elevation angles for an antenna located at 42 degrees, 06 minutes N lat, 76 degrees, 13 minutes W long, pointing towards a geosynchonous ...

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. ...

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.