f + 6 = ?
Well, the answer can be pretty much any number. It all depends on what number f is. f is a variable, so you can plug in any number to replace f.
For example, if f was 5, this is what the equation would look like:
5 + 6 = 11
If f was -3, this is what the equation would look like:
-3 + 6 = 3
So, again, the answer can be anything. It depends on what f is.