$\displaystyle f(x)=x+\sqrt{x}$
Is it even possible to find the inverse of this?
*edit* the question is find $\displaystyle f^{-1} (6)$ but to do that I must 1st find $\displaystyle f^{-1} (x)$
no, you just need to find what value of x gives 6 as an answer,
when they say find $\displaystyle f^{-1}(6)$ they are asking what value of x gives 6 as an output for f(x), and you do not need a formula for $\displaystyle f^{-1}$
Just by looking I can see the answer is 4
$\displaystyle 6=x+\sqrt{x}$