For this you have to folo a rule that says (first, outer, inner, last) basically this says
you take the first (3x) and multiply it out.
3x(2x-4) =6x^2-12x
then you do the same with the second
4(2x-4)= 8x-16
now you have
(3x+4)(2x-4) = 6x^2-12x+8x-16
now you combine like terms.
6x^2-4x-16 is your answer