The two question marks in the problem shown below represent the same number. What is the number?

$\displaystyle\frac{{{9}{{z}}^{{7}}+{15}{{z}}^{{5}}-{12}{z}}}{{{3}{{z}}^{{3}}}}={3}{{z}}^{?}+{5}{{z}}^{{2}}-\frac{{?}}{{{{z}}^{{2}}}}\usepackage{cmbright}\Huge\$