[tex]\left(\dfrac{1}{3}\right)^2\cdot\left(\dfrac{1}{3}\right)^x=\dfrac{1}{729}\\\\\left(\dfrac{1}{3}\right)^{2+x}=\left(\dfrac{1}{3}\right)^6\iff2+x=6\ \ \ \ |-2\\\\\boxed{x=4}\\\\\text{used:}\\\\a^n\cdot a^m=a^{n+m}[/tex]
