Well, first they're assuming that the m<ABC = 90
The value of m<ABC is not given, so they aren't allowed to make up values based on the way the picture looks.
It is given that BD bisects <ABC, which means it splits the angle evenly..
Giving this equation: (x-5) = (2x-6)
Solving for x
-5 + 6 = 2x - x
1 = x
