27m^3+64n^3 answer sheet says it is (3m+4n) (9m^2-12mn+16n^2) can someone tell me where they got the "-12mn" from?

27m^3+64n^3 is an example of the sum of two cubes.

There's a factoring formula for that:  a^3 + b^3 = (a + b)(a^2 - ab + b^2.

Thus,  27m^3+64n^3  = (3m + 4n) ( 9m^2 - 12mn + 16n^2).

Could also find this out by dividing 3m+4n into  27m^3+64n^3.