a)
[tex]3x^{2}-12x = 0 \ \ /:3\\\\x^{2}-4x = 0\\\\x(x-4) = 0\\\\x = 0 \ \vee \ x-4 = 0\\\\x = 0 \ \vee \ x = 4\\\\x \in \{0,4\}[/tex]
b)
[tex]4x^{2}+20x = 0 \ \ /:4\\\\x^{2}+5x = 0\\\\x(x+5) = 0\\\\x = 0 \ \vee \ x+5 = 0\\\\x = 0 \ \vee \ x = -5\\\\x \in\{-5, 0\}[/tex]
c)
[tex]5x^{2}-x = 0\\\\x(5x-1) = 0\\\\x = 0 \ \vee \ 5x = 1 \ \ /:5\\\\x = 0 \ \vee \ x = \frac{1}{5}\\\\x \in \{0,\frac{1}{5}\}[/tex]
