Given f(x) = 4 - x - x², find f(-3)

That's just a easy "plug-in".

f(x) does NOT mean fx.  It means

"Plug in for x on my right side
whatever is in the parentheses and 
simplify"

When you are given an equation for
f(x) and then asked to find 
f(some number) all you do is plug in
that number for x all the way across.
Then simplify the right side.

Here you are given

     f(x) = 4 - x - x²

and then asked to find f(-3), so all 
you do is take

     f(x) = 4 - x - x²

and every where you see an x put (-3) 
in place of it, like this

     f(x) = 4 - x - x²

    f(-3) = 4 - (-3) - (-3)²

Then you simplify the right side

    f(-3) = 4 + 3 + 9

    f(-3) = 16

and you leave it just like that.
   
Wasn't that easy?  This is probably
one of the easiest things you'll
ever study in algebra.

Edwin