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 \begin{aligned} a_{\mathrm{t}} &=\dfrac{\Delta v}{\Delta t} \\ &=\dfrac{30.0 \mathrm{~m} / \mathrm{s}}{4.20 \mathrm{~s}} \\  &=7.14  \mathrm{~m} / \mathrm{s}^{2}.\end{aligned}