ok i know how to do this problem but I'm stuck at the last part.

the problem is :

$\displaystyle f(t)=sqrt(4t+1)$

find $\displaystyle f''(2)$

I found the 1st derivative but i'm stuck at the 2nd derivative! I know that once I find the 2nd F''(x) I will plug the 2 but I'm stuck at the 2nd derivative.

Plz help me get the 2nd derivative and i'm set !!!!

Thanks