Tìm x, biết:

$a) 3x\left(12x-4\right)-9x\left(4x-3\right)=30$

$b) x\left(5-2x\right)+2x\left(x-1\right)=15$

Giải:

$$\begin{gathered} a) 3x\left(12x-4\right)-9x\left(4x-3\right)=30 \\ 36x^2-12x-36x^2+27x=30 \\ 15x=30 \\ x=2 \end{gathered}$$ 

$$\begin{gathered} b) x\left(5-2x\right)+2x\left(x-1\right)=15 \\ 5x-2x^2+2x^2-2x=15 \\ 3x=15 \\ x=5 \end{gathered}$$