i) Find the points on the graph of y=(1/3)x^3 -5x-(4/x) at which the slope of the tangent is horizontal.

ii) find the slope of the demand curve D(p)=20/((sqrt p -1)), p>1, at point (5,10).

I need help with the two questions above, I'm doing practice questions with the slope of tangent but these last two questions I'm stuck on. Please help me or guide me in the right direction so I can finish these two. Thanks for your help in advance!