OKay, so I'm trying to figure this problem out and it's been giving me problems. Yes, it's easy for a lot of people, but me not so much ha ha

(-3xy^4)(-4x^3y^6)

Ok, I'll give you an example, and try to do it.

(2xy)(3xy) = 2\times x\times y \times 3 \times x \times y = 2\times 3 \times x \times x \times y \times y = 6 x^2y^2

See? Re-write to make it simpler, then re group the similar ones for the sake of not confusing yourself, and multiply each of them!

Try it! :)