So far, you have found that . Now you need to differentiate again to find . But remember that this means the second derivative of gwith respect to y. So you need to differentiatewith respect to y. Use the chain rule for that: differentiate it with respect to x and then multiply the result by . You should find that (because ). But (because g is the inverse of f), so you get . All that remains is to trade the variable y for an x.