Clicking on a particular solution step can be the best/quickest way to review the logic.  Notice that '13Z' is being divided into "8Z+5Z" to identify common factors for both the first and last pair of terms ('Z' is common to 5Z and 2Z^2 and '8' is common to 8Z and 20). 

It is also worth noting that variables are case-sensitive, so if 'Z' and 'z' had been entered into the solver then they would be interpreted as two different variables.