Answer
$x=5$
$m\angle ABC=50$
Work Step by Step
Use the definition of a bisector:
$m\angle ABD=m\angle DBC$
$5x=3x+10$
Subtract 3x from each side.
$5x-3x=3x+10-3x$
$2x=10$
Divide each side by 2.
$2x\div2=10\div2$
$x=5$
Angle Addition Postulate:
$m\angle ABC=5x+3x+10=8x+10=8(5)+10=40+10=50$
