\(a,12x^2y^2\left(\dfrac{-3}{4}x^3y\right)=\left(12.\dfrac{-3}{4}\right)\left(x^2.x^3\right)\left(y^2.y\right)=-9x^5y^3\\ b,-x^3y^2.\left(x^2y\right)^3=-x^3y^2.x^6y^3=\left(-x^3.x^6\right)\left(y^2.y^3\right)=-x^9y^5\\ c,-16x^{3-n}\left(\dfrac{-5}{8}ax^{3+n}\right).\left(-2021^n\right)^0=\left(-16.\dfrac{-5}{8}\right)\left(x^{3-n}.x^{3+n}\right).a.1=10x^{3-n+3+n}.a=10ax^6\)
a: \(=-12\cdot\dfrac{3}{4}x^2y^2\cdot x^3y=-9x^5y^3\)
b: \(=-x^3y^2\cdot x^6y^3=-x^9y^5\)
c: \(=-16x^{3-n}\cdot\dfrac{-5}{8}a\cdot x^{3+n}\cdot1=10a\)