$\displaystyle \displaystyle f(x)=\sqrt{x+1}+\frac{b}{x}$

Find $\displaystyle \displaystyle b$ such that $\displaystyle \displaystyle f(x)$ has a point of inflection at $\displaystyle \displaystyle x=3$

Answer in the back has it at $\displaystyle \displaystyle \frac{27}{64}$, but I'm not sure how to do it.