Ugh, #5 is giving me a headache, I can't crack it.
y=(x-y)(2xy)-x is about as far as I get, and it definitely seems like it's gonna be a fourth-grader, so feel free to help me out with this one a bit.

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram