I have a problem:
if z=x+y then f(z)=?
(f(x,y):is given)
please help me

you may not be understanding the terminology properly...
f(z) is a function that is calculated from substituting in the variable z.
f(x,y) is a function that is calculated from substituting in the variables x and y.

If z = x + y
and f(x,y) is given
then f(x,y) = f(z)