3. Given a fractional-linear (neither constant nor linear) function $f(x)=\frac{a x+b}{c x+d}$. It turns out that $f(f(x))=\frac{1}{2} f\left(\frac{1}{2} x\right)$ and $b c=-12$. Find $d$. If there are multiple possible values, list them in any order separated by a semicolon.
Answer: $-2 ; 2 | 2 ;-2$

Input allowed: digits, minus, division sign, period or comma as decimal separator, semicolon

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