Math.

9^x = (1/3)^(x+3)        Take log of both sides

x log 9   = (x+3) log (1/3)

x/(x+3) = log(1/3)/(log 9)

x/(x+3) = - 0.5

x = -.5 x - 1.5

1.5x = -1.5

x = -1