I'm confused as to how to do orthogonal trajectory problems. The current calculus book I have doesn't explain how to do these problems, yet these problems appear in the back of some of the chapters in the book.

Here's an example problem:

Find the value of the number a such that the families of curves $\displaystyle y=(x+c)^{-1}$ and $\displaystyle y=a(x+k)^{1/3}$ are orthogonal trajectories.

How would I go about doing this?