$f(x)=3x+5$, what is $f(\frac{1-x}{x})$? How can I solve this problem? Should I equalize $x=\frac{1-x}{x}$? Or find $f(\frac{1-x}{x})=3(\frac{1-x}{x})+5$?

Since $f(x) = 3x+5$ , taking $\left(\dfrac{1-x}x\right) = y $ , we get :

$$f(y) = 3y+5\implies f\left(\dfrac{1-x}x\right) = 3\left(\dfrac{1-x}x\right) +5$$

$$\color{#24f}{f\left(\dfrac{1-x}x\right) = \dfrac 3x +2}$$