I presume you are looking for two values of x that would suit?

<-- This defines .

<-- This defines r.

So

<-- This defines x.

So solve for x:

Frankly I wouldn't try to fiddle with factoring it, I'd just use the quadratic formula, but as it happens it DOES factor: (which I figured out by using the qf. )

So x = 5 or x = -11/14.

So your two sequences are:

x = 5: 81, 18, 4, ...

x = -11/14: -324/14, 90/7, -25/14, ...

-Dan