Chapter 1 - Tools of Geometry - 1-5 Exploring Angle Pairs - Practice and Problem-Solving Exercises - Page 39: 28

$x=15$ $m\angle ABC=48$

Use the definition of a bisector: $m\angle ABC=2(m\angle ABD)$ Substitute: $4x-12=2(24)$ $4x-12=48$ Add 12 to each side: $4x-12+12=48+12$ $4x=60$ Divide each side by 4: $4x\div4=60\div4$ $x=15$ $m\angle ABC=4x-12=4(15)-12=60-12=48$