we know that two sides are twin sides, so angle that each one makes at the base, is exactly the same angle those we get twin angles at the base as well, and the vertex above is just 180° - the twin angles, in this case 180° - 66° - 66°.
an angle bisector stemming from twin sides, like in this case, will hit the base side perpendicularly always, let's recall that an angle bisector cuts an angle into twin angles.