Identify points on the x-axis by their x-coordinates. For any two positive numbers a and b, prove that the harmonic mean of a and b is the harmonic conjugate of 0 with respect to a and b.

In our example we were given a = 2 and b = 3 and their harmonic mean was calculated as 12/5 or 2 and 2/5. I'm having trouble with the "harmonic conjugate of 0 with respect to a and b" part. Any help please!?