To find $a$ in the equation $f(x) = a - 2x + 1$, additional information is required. This could include:

1. A specific value of $f(x)$ for a given $x$.
2. A condition or property of the function, such as a root (where $f(x) = 0$) or a specific behavior (e.g., maximum, minimum, etc.).

Could you provide more details so I can proceed?

Here are some guiding questions to help you:

1. Do you have a specific value of $f(x)$ for a given $x$?
2. Is there a specific point $(x, f(x))$ that satisfies this function?
3. Is $a$ meant to satisfy a particular condition in the equation?
4. Are we solving for $a$ in a system of equations?
5. Is this part of a larger problem involving $f(x)$?

Tip: Always identify if the problem requires a specific input or condition to solve for unknowns.