For part (a), \((f+g)(x) = 10x + 3\) and the domain is \(\{x \mid x \text{ is any real number}\}\).
For part (b), \((f-g)(x) = 13 - 4x\) and the domain is \(\{x \mid x \text{ is any real number}\}\).
\[
\boxed{(f+g)(x) = 10x + 3, \quad (f-g)(x) = 13 - 4x}
\]