To find the area of a rectangle you must multiply the length by the width. In order to multiply monomials, multiply the coefficients and add the exponents.

$\displaystyle{16}{{z}}^{{6}}\cdot{8}{{z}}^{{3}}={16}\cdot{8}\cdot{{z}}^{{6}}\cdot{{z}}^{{3}}={128}{{z}}^{{{6}+{3}}}={128}{{z}}^{{9}}\usepackage{cmbright}\Huge\$