$\begin{array}{l}\text{Đáp án:}\\x=-4\\\text{Giải thích các bước giải:}\\\left(\dfrac{3}{4}\right)^{x}=\dfrac{2^{8}}{3^{4}}\\⇒\dfrac{3^{x}}{4^{x}}=\dfrac{2^{8}}{3^{4}}\\⇒3^{x} \times 3^{4}=4^{x} \times 2^{8}\\⇒3^{x}\times3^{4}=4^{x}\times2^{(2\times4)}\\⇒3^{x}\times3^{4}=4^{x}\times(2^{2})^{4}\\⇒3^{x+4}=4^{x}\times4^{4}\\⇒3^{x+4}=4^{x+4}\\⇒x+4=0\\⇒x=0-4\\⇒x=-4\\\text{Vậy x = -4}\end{array}$
