@wisyallsip132 

The easiest way I know to solve your problem is as follows:

restart;

f := x -> sqrt(1 + x^2)

maximize(abs(diff(f(x), x $ 2)), x = 0 .. 2)

I am wondering whether you have already solved your problem or not yet.

In case you are still having the issue, you need to visit the spam folder in your email account. You will likely find a good surprise. Many automatic emails go there.