Chapter 5 - Exponential and Logarithmic Functions - 5.3 One-to-One Functions; Inverse Functions - 5.3 Assess Your Understanding - Page 282: 79

$x=-4$

The base is same on the 2 sides of the equation (and it is not 1), hence they will be equal if the exponents are equal. Hence $x=3x+8$, Solve the equation: \begin{align*} x&=3x+8\\ x-3x&=8\\ -2x&=8\\ \frac{-2x}{-2}&=\frac{8}{-2}\\ x&=-4 \end{align*}