1. The problem is to write a given expression or function as $f(x)$, which means expressing it as a function of $x$. 2. The general form of a function is $f(x) = \text{expression involving } x$. 3. To write any expression as $f(x)$, simply replace the expression with $f(x)$ and ensure the right side is in terms of $x$. 4. For example, if you have $y = 2x + 3$, writing it as a function is $f(x) = 2x + 3$. 5. If you provide the specific expression, I can help write it explicitly as $f(x)$. Final answer: To write any expression as a function of $x$, use the notation $f(x) = \text{expression in } x$.