Out= 3+2(x+I y)
f[z_]:= defines a function f that takes a single argument of any kind. That z_ says that you want to name that argument z while inside the definition of the function and the _ says this will match any single expression.
What you had then done is tried to use f[x,y] which looks like a function f that takes two arguments, x and y. But you had not defined a function f that accepted two functions, so it did nothing.
Read through this
and see if it makes a little more sense