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I've tried multiple times so I must be doing something wrong. I differentiated to get $ dy/dx = 6x^2 - 18x - 12 $ and then I set that equal to $12$ and rearranged/factorised to get $x = 4$ and $x = -1$.

The answer is (according to the back of the book): $(-1,8)$ and $(4,-57)$.

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You are on the right track. Once you have found the x-values ($ -1$ and $4$), plug those (separately) into the original equation and solve for the corresponding y-coordinates.

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you must solve the equation $$f'(x)=6x^2-18x-12=12$$ for $x$

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The question asked for the coordinates, not the x values.

You have the correct x values. Now find f(x) for each to get the coordinate pairs (x, f(x)).

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