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If the formula is x(t) = 16*t - 3*t^2, then the answer is
x'(t) = 16 - 6t =====> x'(t) = 0 when 16-6t = 0 ====> t = = seconds.
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I'd say that the post is written in an unreadable way.
I need to guess what is written there . . .
It is not the method to prepare the posts to this forum.