Fixing individual keys is not usually possible, but a complete new keyboard is not that expensive and quite easy to fit. Make sure you get one of the right nationality (US, UK etc as appropriate) and do a google search for Asus X200CA keyboard replacement for a guide. From a quick glance it looks like there are one or two screws on the bottom of the laptop that you have to remove, then there are a few tabs, probably along the top of the keyboard, that you have to release with a spatula or thin knife to allow the keyboard to be lifted out.