The function $f(x) = -x + 4$ is a linear function. To find the reflection of this function across the x-axis, we need to negate the entire function.

For a reflection across the x-axis, the transformation is given by: $g(x) = -f(x)$ Thus, for $f(x) = -x + 4$, we negate it: $g(x) = -(-x + 4) = x - 4$

Therefore, the equation for $g(x)$, the reflection of $f(x)$ across the x-axis, is: $g(x) = x - 4$

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Related Questions:

1. What is the effect of reflecting a function across the y-axis?
2. How do transformations like shifts and stretches affect the graph of a function?
3. What is the geometric interpretation of the slope in a linear function?
4. How would reflecting the function $f(x) = 2x - 5$ across the x-axis change its equation?
5. What does the y-intercept represent in the context of a linear function?

Tip: When reflecting functions across the x-axis, simply negate the output of the function. This is useful for understanding the symmetry of functions.