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

c) $x^4+9x^2=6x^3$

$x^4+9x^2-6x^3=0$

$x^4-6x^3+9x^2=0$

$x^2\left(x^2-6x+9\right)=0$

$x^2\left(x^2-2\cdot x\cdot 3+3^2\right)=0$

$x^2{\left(x-3\right)}^2=0$

$\begin{array}{ccc}{x}^2=0& \text{lub}& {\left(x-3\right)}^2=0\\ x=0& & x-3=0\\ & & x=3\end{array}$

Odp.: $x\in \left\{0,3\right\}$