Precalculus (6th Edition) Blitzer

$f(x)$ means $f$ of $x$ or the value of the function $f$ at $x$.
$f(x)$ does not mean $f$ times $x$. $f(x)$ means $f$ of $x$ -- the value of a function $f$ at $x$. For example: $f(x) = x+2$. $f(2)$ means the value of the function when x = 2. So $f(2) = 2+2 \\f(2) = 4.$