solving for triangles using the law of sines

Hi,

I recently did the exercise problems for this section with general triangles (not right angles). 

The question asks for angle B and adds that angle B is obtuse. I used the law of sines and then took the inverse sine to get 77 degrees.

I believed this should have been the correct answer, yet it was not an obtuse angle.

Khan Academy said it was the angle measure for Angle C and that somehow I should have known this.?????????????????????????????????????????????!!

How can the answer for angle B magically become angle C? 

No explanation that I could understand was offered. Please explain.

oohmtree