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\begin{aligned} &D(3 f(x))=3 D(f(x))=3(6)=18, \quad D(x)+g(x))=D(f(x))+D(g(x))=6+0=6 \\ &D(f(x)-g(x))=D(f(x))-D(g(x))=6-0=6 \\ &D(f(x) g(x))=f(x) D(g(x))+g(x) D(f(x))=(6 x+8)(0)+(2)(6)=12 \\ &D(f(x) / g(x))=\frac{g(x) D(f(x))-f(x) D(g(x))}{(g(x))^{2}}=\frac{(2)(6)-(6 x+8)(0)}{2^{2}}=\frac{12}{4}=3 \end{aligned}