Answer:
[tex]x=80^{\circ}[/tex]
Step-by-step explanation:
The answer is correct and the reasoning is fine. I'd mention that that [tex]\angle x[/tex] corresponds with angle [tex]\angle ABC[/tex] and since [tex]\angle ABC[/tex] and [tex]\angle CBD[/tex] form one side of a line, they are supplementary (add up to 180 degrees). Therefore, [tex]x=180^{\circ}-100^{\circ}=\boxed{80^{\circ}}[/tex].Your reasoning is correct as well, but maybe just add on and explain that [tex]m\angle B=m\angle CBD[/tex] because they are vertical angles.
In addition, I have a hard time believing a computer will be able to grade the reasoning portion of the question well. It's entirely possible it isn't giving you credit for that because it needs to be manually graded.
