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If we know that $(f\circ f)(x)=4x+3$, with $f(0)=4$, what is $f(3)=?$

I have found that $f(f(x))= 16x+15$, but I don't know where to go from there.

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    $\begingroup$ This is not clear. What's the difference between $f\circ f(x)$ and $f[f(x)]$? $\endgroup$ – lulu Mar 26 '18 at 19:12
  • $\begingroup$ I don't understand the part about $16x+5$. For the earlier part, note that $f\circ f(0)=3\implies f(f(0))=3\implies f(4)=3\implies f\circ f(4)=f(3)$. Can you finish from there? $\endgroup$ – lulu Mar 26 '18 at 19:16
  • $\begingroup$ how did you find that f(4)=3 $\endgroup$ – Lyds Mar 26 '18 at 19:23
  • $\begingroup$ We know that $f(f(0))=3$ and $f(0)=4$. $\endgroup$ – lulu Mar 26 '18 at 19:23
  • $\begingroup$ so how does that help us? $\endgroup$ – Lyds Mar 26 '18 at 19:32
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Notice that $f(0)=4$ and $f(f(0))=3$, so $f(4)=3$ which implies $f(3)=f(f(4))=19$.

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