# Self-Inverse Function?

Algebra Level pending

Let $$a, b, c$$ be integers such that the function $$\ f(x) = \large \frac{ ax-b} { cx} \$$ has the following properties:

• $$f(1) = -2.5$$
• $$f(5) = -0.5$$
• $$f(f(x) ) = x$$

Find $$f(-20)$$.

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