Sequence

3^2 , x, 3^4*3^2   =

3^2 , x  , 3^6

Since this is arithmetic, the difference between successive terms is  constant....so.....

x - 3^2  =  3^6 - x    simplify

2x =  3^6 + 3^2

x  =  [ 3^6 + 3^2 ] / 2

x = 738 / 2

x = 369