6.4. Let $f(x)=x^{2}+p x+q$. It is known that the inequality $|f(x)|>\frac{1}{2}$ has no solutions on the interval $[4 ; 6]$. Find $\underbrace{f(f(\ldots f}_{2017}\left(\frac{9-\sqrt{19}}{2}\right)) \ldots)$. Round your answer to the nearest hundredth if necessary.
Answer: 6.68 (exact value: $\frac{9+\sqrt{19}}{2}$ ).