When is y = 2/(4-x) concave downwards?

[karisrou]

17. For what value of x is the graph of y = 2 / (4 - x) concave downwards?

I found the first derivative = 2/(4-x)^2

And then the second 4/(4-x)^3

But I think I might have messed up somewhere in there

 

[futurebird]

You're not doing the derivatives correctly. You need to use this rule:

http://archives.math.utk.edu/visual.calculus/2/quotient_rule.4/index.html

\left( \frac{f}{g} \right)' = \frac {gf' - fg'}{(g)^2}