Given that $f(x)=|x|$ and $g(x)=3 x + 1$, calculate
(a) $f\circ g(x)$= , its domain is
(b) $g\circ f(x)$= , its domain is
(c) $f\circ f(x)$= , its domain is
(d) $g\circ g(x)$= , its domain is
Note: See interval for help entering intervals.