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Inflection points with second derivative: find the error

Problem

Jenny was asked to find where f(x)=x4 has inflection points. This is her solution:
Step 1: f(x)=4x3
Step 2: f(x)=12x2
Step 3: The solution of f(x)=0 is x=0.
Step 4: f has inflection point at x=0.
Is Jenny's work correct? If not, what's her mistake?
Choose 1 answer: