Show that f(x,y)=x+y (with |x|=|y|) isn't a one way function
A function $f$ is one-way if given $f(z)$ for random $z$, it is hard to find an input $w$ such that $f(w) = f(z)$. So in order to show that $f$ isn't one way, you need to show that given $f(z)$ for random $z$, it is not hard to find an input $w$ such that $f(w) = f(z)$. Good luck!