Explanation:
since this is a parallelogram, angle B = angle D.
therefore,
12x + 13 = 9x + 40
3x + 13 = 40
3x = 27
x = 27/3 = 9
and so,
angle B = 12×9 + 13 = 108 + 13 = 121°
don't get confused by the diagram, where angle B and D look like they are smaller than 90°.
but given the specified information that is not true and the graphic is simply misleading in that regard.