Thanks everybody. When I discussed this with the teacher previously, she didn't have any interest in considering a different answer. She believes any other answer than subtraction means he is less prepared for harder second grade math, as this was a first grade review lesson. Not sure if she believes she taught him something more advanced? I don't think this is the case? At very least, she could have acknowledged that giving him a page with an example other than what she considers correct is misleading, but she seemed unaware or didn't care.
It's just a question on a homework assignment, so it's not really a big deal in the grand scheme of things. But her response was very off-putting, and I have some concerns about how the material is being taught and her attitude toward my son. His grades now are just on grade level. For exemplary, she has stated that a 100% or close to it is needed on second grade testing, not sure if that includes homework. Above grade level knowledge or aptitude isn't a factor, so the grading becomes meaningless and irrelevant unfortunately.