One of the product rules for exponents was used.
[tex](ab)^{c}=a^{c}b^{c}[/tex]
Or, you can consider that the definition of an exponent and the associative and commutative properties of multiplication were used.
[tex](3z)^{2}\\=(3z)(3z)\\=3\cdot 3\cdot z\cdot z\\=3^{2}z^{2}\\=9z^{2}[/tex]
