A function $f$ is the inverse of a function $g$ if $$f(g(x)) = x$$ for all $x$ in the domain of $g$. For example, $f(x) = \frac{x+1}{2}$ is the inverse of the function $g(x) = 2x-1$ since $$f(g(x)) = f(2x-1) = \frac{(2x-1)+1}{2} = x.$$
Functions do not have an inverse if several values of $x$ give rise to the same value of $f(x)$. For example the function $f(x)=x^2$ does not have an inverse.
To compute the inverse of a function $g$ you solve the equation $y=g(x)$ for $x$ and then reverse the role of $x$ and $y$. In the above example we solve $y=2x-1$ for $x$ which gives $x=\frac{y+1}{2}$. Interchanging $x$ and $y$ gives the inverse function of $g$ which is $f(x) = \frac{x+1}{2}$.
Consider now the function $$g(x) = 3x+4.$$ Its inverse function is
$f(x) =$ .
Hint: Proceed as in the example.