Rectilinear motion (differential calc)

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A particle moves along the x-axis so that at any time t, is greater than or equal to, 0 its velocity is given by v, left parenthesis, t, right parenthesis, equals, 3, t, start superscript, 4, end superscript, minus, 3, t, start superscript, 3, end superscript, plus, 4.
How many times is the particle's acceleration equal to zero?
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