I think I get the concept of the whole thing at least for the easy ones. When it comes to ones like:

If x+y=x where x,y and z are postive, simplify 2{x-z} + {z-x}

I get entirely lost. Can anyone help me with this and break it down step by step so that I can understand??

Thank you and I want you to know that your help is greatly apreciated.