$6m^2n * 0,4mn^3 = (6 * 0,4) * (m^2 * m) * (n * n^3)$

$7\frac{1}{3}b^3c^2 * \frac{9}{11}a^4b^5 = (\frac{22}{3} * \frac{9}{11}) * a^4 * (b^3 * b^5) * c^2 = (\frac{2}{1} * \frac{3}{1})a^4b^8c^2 = 6a^4b^8c^2$

$-5x^4y^2z^8 * (-0,8x^6y^8z^2) = (-5 * -0,8) * (x^4 * x^6) * (y^2 * y^8) * (z^8 * z^2) = 4x^{10}y^{10}z^{10}$

$-5\frac{3}{7}abc * 3,5a^{12}b^{10}c = (-\frac{38}{7} * \frac{7}{2}) * (a * a^{12}) * (b * b^{10}) * (c * c) = (-\frac{19}{1} * \frac{1}{1})a^{13}b^{11}c^{2} = -19a^{13}b^{11}c^{2}$