If f is one-to-one, twice differentiable fucntion with inverse function g, show that

$\displaystyle g''(x) = -\frac{f''(g(x))}{[f'(g(x))]^3}$

What is this asking me to do?

Also deduce that if f is increasing and concave upward, hten its inverse function is concave downwards. (But I need to understand the top part before worrying about this)

I'm so lost...