You may have thought that the exponent of z is 0, or you may have known that it is 1 and thought you should multiply the exponents. Instead, you should apply the product rule, which says $\displaystyle{{x}}^{{a}}\cdot{{x}}^{{b}}={{x}}^{{{a}+{b}}}\usepackage{cmbright}\Huge\$ . That is, to multiply factors with like bases you add the exponents.