ไปเอามาจากไหนครับ คร่าวๆนะครับ
from $f(f(x))=x\rightarrow f$ is injective
and $\displaystyle f\Big(\frac{b}{d}\Big)=f\Big(\frac{-b}{a}\Big)=0$ so $d=-a$
Put $x=19,97$ respectively we get $\displaystyle c=\frac{a}{58}..(*)$ Let $(*)$ put ti fine $b=\frac{19^2}{58}a-38a$
So $$f(59)=\frac{a\Big(59+\frac{19^2}{58}-38\Big)}{a\Big(\dfrac{59}{58}-1\Big)}=21\cdot 58+19^2$$
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