I've tried every which way I can think of to manipulate the algebra using the various laws I was given, but I cannot figure out a way to get $\overline{x+y}$ to convert to only NAND operations using $\overline{x\cdot x}$, $\overline{x\cdot y}$, or $\overline{y\cdot y}$
I've tried using idempotent, DeMorgan's, identity, and double negation laws to no avail.