Question 126813
y = x^3 – 3x^2 
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Definition: x=c is a critical point of the function f(x) if
1st f(c) exists 
AND
f'(c)=0  OR f'(c) does not exist
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Your Problem:
f'(x) = 3x^2-6x
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Check f'(x)=0
3x^2-6x = 0
3x(x-2)=0
x = 0 or x = 2
These are potential critical values.
Check f(0) to see if it exists; it does
Check f(2) to see if it exists; it does
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Conclusion: x=0 and x=2 are critical values of f(x)
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Cheers,
Stan H.