The quadrilateral below is a parallelogram. Determine the value of x.
angle BDC = 15
angle DAB = 136
opposite angles in a parallelogram are congruent, so therefore angle DCB is equal to DAB, and is 136 degrees.
the sum of the interior angles of a triangle is 180 degrees, so following that, angle x = 180 - 136 - 15 (since they are all angles in triangle DCB).
so, angle x is 29 degrees.
hope this helped! if you would like proof for any of the properties i mentioned in my solution, feel free to ask!