b) $24x^4-81x=0|:3$

$8x^4-27x=0$

$x\left(8x^3-27\right)=0$

$\begin{array}{c}\begin{array}{ccc}x=0& \text{lub}& 8x^3-27=0\\ & & 8x^3=27|:8\\ & & x^3=\frac{27}{8}\\ & & x=\frac{3}{2}\\ & & x=1\frac{1}{2}\end{array}\end{array}$

Odp.: $x\in \left\{0,1\frac{1}{2}\right\}$