trouble understanding free varibles

hey guys

slighlty confused with understanding free variables

for example

x=y+z

y=z

woud z be the free variable? if so why???

and heres an example i made up

x=-2y+z

y=z

5x= w

what are the free variables? z and w?? why?

would this be a good stragety to follow

say u have x=y+z

y=z like in the first example....

cross out all the variables which have an x and a y, since they are definined.. and the reamining none crossed out terms are the free variables ?

thanks guys and gals