Well, 3^9 is the same thing as
3 x 3 x 3 x 3 x 3 x 3 x 3 x 3 x 3
So 3^9 is equal to 19,683
For me personally, the easiest way to do this is by isolating the x² term, and finding the square root of both sides. The hardest way (well actually, the longest way) would be to use the quadratic formula. It just complicates things unnecessarily.
The next step in the series is choice D.