Are a and b the instance names of the TextFields? If so,
using a+b doesn't really make sense. You probably wanted to use
a.text and b.text. But even using a.text+b.text won't give you the
numerical sum because both of those are strings. And when strings
are added, they get concatenated, e.g. "123"+"12" = "12312". So, we
need to first convert these two into Numbers. And we can do that
using the Number function, so then we get:
Number(a.text)+Number(b.text);