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\left.\mathbf{D}\left(\left(x^{2}+1\right)(7 x-3)\right)=\left(x^{2}+1\right) \mathbf{D}(7 x-3)+(7 x-3)\right)\left(x^{2}+1\right)=\left(x^{2}+1\right)(7)+(7 x-3)(2 x)=21 x^{2}-6 x+7