Multiply :

3 - $\dfrac{2}{3}$ xy + $\dfrac{5}{7}$ xy² - $\dfrac{16}{21}$ x²y by - 21x²y²

$\Big(3 - \dfrac{2}{3}xy + \dfrac{5}{7}xy^2 - \dfrac{16}{21}x^2y\Big) \times (-21x^2y^2)\\[1em] = \Big(3 \times (-21x^2y^2) - \dfrac{2}{3}xy \times (-21x^2y^2) + \dfrac{5}{7}xy^2 \times (-21x^2y^2) - \dfrac{16}{21}x^2y \times (-21x^2y^2)\Big)\\[1em] = \Big(-63x^2y^2 - \dfrac{2 \times (-21)}{3}x^{1+2}y^{1+2} + \dfrac{5\times (-21)}{7}x^{1+2}y^{2+2} - \dfrac{16 \times (-21)}{21}x^{2+2}y^{1+2}\Big)\\[1em] = -63x^2y^2 - \dfrac{-42}{3}x^3y^3 + \dfrac{-105}{7}x^3y^4 + \dfrac{16 \times \cancel{(21)}}{\cancel{21}}x^4y^3\\[1em] = -63x^2y^2 + 14x^3y^3 - 15x^3y^4 + 16x^4y^3$

Hence, (3 - $\dfrac{2}{3} xy + \dfrac{5}{7} xy^2 - \dfrac{16}{21}$ x²y) x (- 21x²y²) = -63x²y² + 14x³y³ - 15x³y⁴ + 16x⁴y³