It's very similar to the Segment Addition Postulate. m<ABC=m<ABD + m<DBC. An angle is a figure that's formed by two rays with a common endpoint called a vertex. (the vertex will always be in the center of your angle).
m<ABD = 37 degrees and m<ABC = 84 degrees. Find m<DBC. It might help if you draw a picture. *Remember, BD bisects (or divides) <ABC*
m<ABC=m<ABD + m<DBC
84 degrees = 37 degrees + m<DBC
47 degrees = m<DBC
Now try one on your own:
m<XYZ = 121 degrees and m<XWY = 59 degrees. Find m<YWZ. Drawing a picture will help.