Does anyone know how to write a FORMAL PROOF for (For all real numbers x)(-(-x)=x)

using the definition (For all real numbers x)(-x = 0-x)

So i need to obviously use the axioms of arithmitec etc... any1 can help me do it? it should take about 20 steps not much less but shouldntbe greater than 20

http://www.math.rutgers.edu/~sussmann/papers/hmw10.pdf is where the axiom rules can be foudn ont he last part of the page