f(x)= x - 3 g(x)= x^2 - x

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f(x) = x^2-x.  Given x - 3g(x) = x^2-x. Given.  Now, subtract x from both sides and divide by -3 to get g(x) = -x(x-2)/3.  You can substitute your answer back in for g(x) to get f(x).