Answer
$y'=-sec(1/x+7x)tan(1/x+7x)\times (-1/x^2+7)$
Work Step by Step
First, find the derivative of y and u. Don't forget to apply chain rule for y':
$y'=-secutanu\times u'$
$u'=-1/x^2+7$
Then plug in u and u' into y':
$y'=-sec(1/x+7x)tan(1/x+7x)\times (-1/x^2+7)$
