how do you get 3x^2 from 2*d/dx(x^3)

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Hmm, I don't believe you do. d/dx(x^3) = 3x^2, so if you have 2*d/dx(x^3) it should be 2*3x^2 = 6x^2. This topic shows this being done toward the end of problem two: http://www.brightstorm.com/d/math/s/calculus/u/the-derivative/t/derivatives-of-polynomial-functions