\(y'=2021\cdot cos\left(x\sqrt{x}\right)^{2020}\cdot\left(cos\left(x\sqrt{x}\right)\right)'\)
\(=2021\cdot\left(-x\sqrt{x}\right)'\cdot sin\left(x\sqrt{x}\right)\cdot cos\left(x\sqrt{x}\right)^{2020}\)
\(=-2021\cdot\dfrac{\left(x^3\right)'}{2\sqrt{x^3}}\cdot sin\left(x\sqrt{x}\right)\cdot cos^{2020}x\sqrt{x}\)
\(=-2021\cdot\dfrac{3x^2}{2x\sqrt{x}}\cdot sin\left(x\sqrt{x}\right)\cdot cos^{2020}x\sqrt{x}\)
\(=-\dfrac{6063}{2}\sqrt{x}\cdot sin\left(x\sqrt{x}\right)\cdot cos^{2020}x\sqrt{x}\)