Factorise the expressions below, taking out as many factors as possible.

[tex]6x+3x^{2} -3x^{3}[/tex]
[tex]2a^{2} b+4ab^{2} -6ab[/tex]
[tex]2x^{2} -3gx+4x^{2}[/tex]

Note: There are already answers attached to the lesson, and you do not have to answer all of the questions. I just want to know HOW to answer these questions. Like the formula or something. Thank you!