a)
[tex]x(3x+2)=3(x^2-4)\\\\3x^2+2x=3x^2-12\\\\3x^2+2x-3x^2=-12\\\\2x=-12 \ \ /:2\\\\\huge\boxed{x=-6}[/tex]
b)
[tex]6(1+2x^2)=4x(1+3x)\\\\6+12x^2=4x+12x^2\\\\12x^2-4x-12x^2=-6\\\\-4x=-6 \ \ /:(-4)\\\\\huge\boxed{x=1,5}[/tex]
c)
[tex]3x(x-5)=(3x+2)(x-1)\\\\3x^2-15x=3x^2-3x+2x-2\\\\3x^2-15x-3x^2+3x-2x=-2\\\\-14x=-2 \ \ /:(-14)\\\\\huge\boxed{x=\frac{1}{7}}[/tex]
