Answer
$x=12$
$m\angle ABC=112$
Work Step by Step
Use the definition of a bisector:
$m\angle ABD=m\angle CBD$
$3x+20=6x-16$
Subtract 3x from each side:
$3x+20-3x=6x-16-3x$
$20=3x-16$
Add 16 to each side:
$20+16=3x-16+16$
$36=3x$
Divide each side by 3:
$36\div3=3x\div3$
$12=x$
Angle Addition Postulate:
$m\angle ABC=(3x+20)+(6x-16)=9x+4=9(12)+4=108+4=112$
