In the $2-$to$-1$ MUX, w$1$ is a selection signal and the two inputs of the MUX could be functions of w$2$ and w$3.$

You can start with analyzing the truth table in two$-$fold: $1)$ what's the output f when w$1=0$; and $2)$ what is the output f when w$1=1,$ both of which could be functions w$.$r$.$t$.$ w$2$ and w$3.$