12. Function $f(x): f(x)=-2 x+4$ Inverse $f^{-1}(x)$ : 13.

Step 1 ：Given the function \(f(x) = -2x + 4\)

Step 2 ：To find the inverse of the function, we switch the x and y in the original function, resulting in \(x = -2y + 4\)

Step 3 ：Solving for y, we get \(y = 2 - \frac{x}{2}\)

Step 4 ：Thus, the inverse function of \(f(x) = -2x + 4\) is \(f^{-1}(x) = 2 - \frac{x}{2}\)

Step 5 ：\(\boxed{f^{-1}(x) = 2 - \frac{x}{2}}\) is the final answer