How to show labels of overlapping points in the same layer in QGIS?

I will assume that you are having trouble with the intended meaning of the question.

We have $f(8)=3$ and $f(35)=6$. Since $f(x)$ is continuous on our interval, if follows by the Intermediate Value Theorem that for any $b$ with $3\lt b\lt 6$, there is an $a$ with $8\lt a \lt 35$ such that $f(a)=b$.

You are being asked to show that the Intermediate Value Theorem holds in this specific situation without using the IVT. Effectively, you are being asked to express the required $a$ in terms of $b$, and to verify that it is indeed between $8$ and $35$.

So we want $\sqrt{a+1}=b$. Now you can take over.