Answer
$y' = -\dfrac{\sin(\ln x)}{x}$
Work Step by Step
In order to derivate this function you have to apply the chain rule
Let's make a «u» substitution to make it easier
$f(u) = \cos(u)$
$u =\ln x$
Derivate the function:
$f'(u) = -u' \sin(u)$
Now let's find u'
$u' = \dfrac{1}{x}$
Then undo the substitution, simplify and get the answer:
$y' = -\dfrac{\sin(\ln x)}{x}$