The definition of an angle bisector is a bisector that cuts an angle into two equal halves.
So BD bisects ∠ABC, therefore ∠ABD = ∠DBC
We can set the expressions inside them equal to each other, solve for x, and substitute it into the expressions again to find the total value of the angle.
4x - 15 = 3x + 12
- 3x - 3x
______________
x - 15 = 12
+ 15 + 15
______________
x = 27
So x = 27, we can plug this back into an expression and do the same with the second one and add them, or, because we know they are equal, just multiply two once we find the value of the first one.
4(27) - 15
= 108 - 15
= 93
Multiply 93 by 2.
93 x 2 = 186
Therefore m∠ABC is 186°
Hope this helps! :)
