3^(x+2) + 9*9^x - 810 =0
9*3^x + 9*3^(2x) - 810 =0
(3^x)=t
9t+9*t^2 - 810 =0
t^2 + t -90 =0
D=361=19^2
t1=-10 t2=9
3^x = 9 x1=2
3^x = -10 x2=∅