Chapter 9 - Review - Page 598: 119

$x=-4$

Express both sides of the equation with the same base. Once the bases are the same, then equate the exponents. That is, \begin{array}{l} 3^x=\dfrac{1}{81}\\\\ 3^x=\dfrac{1}{3^4}\\\\ 3^x=3^{-4}\\\\ x=-4 .\end{array}