Given the function $-f(x) = 5x - 1$, you can solve for $f(x)$ by multiplying both sides of the equation by $-1$:

$f(x) = -5x + 1$

So, the function $f(x)$ is:

$f(x) = -5x + 1$

Would you like more details or have any questions about this?

Here are some related questions:

1. What is the slope of the function $f(x) = -5x + 1$?
2. How would you graph the function $f(x) = -5x + 1$?
3. What is the y-intercept of the function $f(x) = -5x + 1$?
4. How does the function $f(x) = -5x + 1$ compare to the function $g(x) = 5x - 1$?
5. What happens to $f(x)$ as $x$ increases or decreases?

Tip: The slope of a linear function gives you the rate of change, telling you how the function's output changes with each unit increase in the input.