Find dy/dx by implicit differentiation and evaluate the derivative at the given point.
xy = 35

So, we have xy=35
Since that, the first thing you should do is to differentiate with respect to x
(look at the pic. 1), as you can see this is simply 1, which means you have to simplify 
Then we need to perform solving for  for dy/dx as we are about to get the derivative of y with respect to x. pic.2
Get back to the oroginal function, now we can substitute this for y to get the func which will depend only on x pic.3
Then pic.4