The answer is correct and the reasoning is fine. I'd mention that that corresponds with angle and since and form one side of a line, they are supplementary (add up to 180 degrees). Therefore, .Your reasoning is correct as well, but maybe just add on and explain that 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.