I’d change the x-intercept input to a Math Response instead of Free Response, then it’ll be easier to give feedback. What do you want that student feedback to look like? There are a lot of ways people choose depending on the complexity. I’ve used a simple 
or 
as a suffix in the input. It’s also pretty common to have a message in a note with a more encouraging statement, or even more specific feedback (e.g. “
Good job finding the y-intercept! Let’s find the x-intercept.”).
Check out this thread specifically about validating standard form of a linear equation. Definitely helpful to search for your issue. Common topics means it may have been addressed. You can also find some great bespoke lessons from others!