I think you have made some mistakes in posting your question, as there is nothing to show in the first part and no mention of k. It would also help us to see exactly what you need help with if you were to post what you have tried so far.
This is a three-part question:
a. the graph y = f(x) in the xy-plane has parametrization x=x, y=f(x), and vector formula r(x) = xi + f(x)j. Use this to show that if f is twice-differentiable, then
b. use the formula for k in part a to find the curvature of y = ln(cosx) when -pi/2 < x < pi/2.
c. show that the curvature is zero at the point of inflection.