6. Let the sum of the digits of a positive integer $a$ be $b$, the sum of the digits of $b$ be $c$, and so on. Eventually, a single-digit number will be obtained, denoted as $f(a)$ (for example: $a=1996, b=1+9+9+6=25, c=2+5=7$, thus $f(a)=7$). Now, let $x>3$ and both $x$ and $x+2$ be prime numbers, $A=1996 x(x+2)$, then $f(A)=$ $\qquad$.
6. 2