Hi i've been struggling over this question for a while. Can anybody help me with this?

Question

A body's velocity at the time t sec is v = 2t^3 - 9t^2 +12t - 5 m/sec. Find the body's speed each time the acceleration is zero.
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What i have been doing is differentiating the finction to find the acceleration with respect to time and setting that equation equal to zero. Once i did that i solved for t(time) and plugged it into the equation that they have in the problem (speed with respect to time).

What am i doing wrong?