a) 21x³y²( -4/7 x); б) -0,25a264 (-8ba").

a) $$21x^3y^2 \cdot \left(-\frac{4}{7}x\right) = 21 \cdot \left(-\frac{4}{7}\right) \cdot x^3 \cdot x \cdot y^2 = -\frac{21 \cdot 4}{7} \cdot x^{3+1} \cdot y^2 = -\frac{84}{7}x^4y^2 = -12x^4y^2$$

б) $$-0,25a^2b^4 \cdot (-8ba^5) = -0,25 \cdot (-8) \cdot a^2 \cdot a \cdot b^4 \cdot b^5 = 2a^{2+1}b^{4+5} = 2a^3b^9$$

Ответ: a) $$-12x^4y^2$$, б) $$2a^3b^9$$