Explanation:
multiplied exponents are added.
like x² × x³ = x⁵
so, up there we have
(-2x³y⁴)(-3x⁴y⁴).
as all operations (except for the exponents) are simms multiplications, this results in one longer expression.
(-2)x³y⁴(-3)x⁴y⁴.
we can unite terms of the same base unit.
-2 × -3 = 6
x³x⁴ = x⁷
y⁴y⁴ = y⁸
so, we get
6x⁷y⁸