to find the second derivative, you simply take the first derivative, then from that result do the exact same thing to get the second.
Remember the derivative of a function f(x) is another function (The 'Gradient function'), denoted f '(x), so all you do is take the derivative of this function to get the 'second derivative,' f''(x)....so it's as easy as that - differentiate twice. Think you can do it now?
edit: beaten to the mark