Answer :
f:R->R, f(x) = 3x+1;Let be g the inverse of f;3x+1 = y => 3x = y-1 => x = (y-1)/3;g:R->R, g(y) = (y-1)/3;
[tex]f(x)=3x+1\to y=3x+1\\\\3x+1=y\ \ \ \ \ |-1\ both\ sides\\\\3x=y-1\ \ \ \ \ |:3\ both\ sides\\\\x=\frac{1}{3}y-\frac{1}{3}\\\\Answer:f(x)=\frac{1}{3}x-\frac{1}{3}[/tex]