Simplify logic function
Your wolfram alpha simplification really simplifies nothing (and all it does was inelegantly apply de Morgan's rules, which you really should be aware of, anyway).
As this is  homework, only a hint: Consider \$a=\overline{x_1}, b=\overline{x_2}, c=\overline{x_3}, d=\overline{x_4}\$. Great, now you have four different things that you combine (with \$\cdot\$).
How many different 2-sets can you make out of four different inputs? What does that tell you about your \$f\$?