let the father's present age be "f" and the son's present age be "s".
so f = 3s (At present the age of father is triple the age of son)
and s + 9 = (1/2)(f + 9)
(Since After nine years the age of the son is half of the age of the father)

=>s + 9 = (1/2)(f + 9)
=>2s + 18 = f + 9
=>2s + 18 = 3s + 9 (Since f = 3s)
=> s = 9 Years.
so f = 27 years.

Now we want "What was the age of father when the son was born"
This means the age of the father 9 years back and that is 18 years.