what is the correct answer of (3a+b)(-2a-4b) using foil method

FOIL stands for First, Outside, Inside, Last.  It is an
algorithm used for multiplying binomials.

(3a + b)(-2a -
4b)

Product of the First
terms:  3a * -2a = -6a^2

Product of the
Outside terms:  3a * -4b =
-12ab

Product of the Inside
terms:  b * -2a = -2ab

Product of the
Last terms:  b * -4b =
-4b^2

-6a^2 + -12ab + -2ab +
-4b^2

Add like
terms.

-6a^2 + -14ab +
-4b^2

or

-6a^2
- 14ab - 4b^2