Gah. Struggling over this for the past 20 minutes...
Find the slope of the line tangent to the curve y=1/x at the point (2,0.5).
This is just a simple application of the power rule:
y(x) = 1/x = x^-1
y'(x) = (-1)x^-2 = -1/x^2
So, y'(2) = -1/4