$\text{a)}{x}^6:x^4=x^{6-4}=x^2$  
 
$\text{b)}{y}^2:y=y^{2-1}=y^1=y$

$\text{c)}{z}^5:z^0=z^{5-0}=z^5$

$\text{d)}{\left(-a\right)}^4:\left(-a\right)={\left(-a\right)}^{4-1}={\left(-a\right)}^3$

$\text{e)}{\left(-b\right)}^3:{\left(-b\right)}^0={\left(-b\right)}^{3-0}={\left(-b\right)}^3$

$\text{f)}{\left(-c\right)}^5:{\left(-c\right)}^5={\left(-c\right)}^{5-5}={\left(-c\right)}^0=1$