Take an input, a formula, and do f(x)
TI BASIC, 6 bytes
This might change, since the rules are still not totally stable, but I wanted to post this while I could.
:Prompt X,F
:F

Asks for X, then parses whatever formula is put into F and displays it.
If I could assume that X was already stored in the memory, it would be
TI BASIC (maybe), 0 bytes

Is this ok?Guess not. :-(