Factor out common factors from top and bottom.
[tex]\frac{12x^6y^{-4}z^2}{3x^2y^{-6}z^3} = \frac{(3x^2y^{-4}z^2)(4x^4)}{(3x^2y^{-4}z^2)(y^{-2}z)} = \frac{4x^4}{y^{-2}z} [/tex]
You can also write it with only positive exponents:
[tex]\frac{4x^4 y^2}{z} [/tex]
