6. Given the function $f(x)=x^{3}-6 x^{2}+17 x-5$, real numbers $a, b$ satisfy $f(a)=3, f(b)=23$, then $a+b=$
Given $f(x)=(x-2)^{3}+5(x-2)+13$, then the graph of $f(x)$ is symmetric about the point $(2,13)$, and $f(x)$ is monotonically increasing, so $f(x)+f(4-x)=26 \Rightarrow a+b=4$.