Answer
$n\approx2.858$
Work Step by Step
As $f(n)$ is given, this is the output value therefore we have to calculate the input value of $n$.
$f(n)=3.1- 2\times\ln n$
$1=3.1- 2\times\ln n$
$-2.1=- 2\times\ln n$
$1.05=\ln n$
$e^{1.05}=e^{\ln n}=n$ (Let both sides be exponents of the base e)
$n\approx2.858$
