Calculating average acceleration

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I have the question "A train accelerates from rest to a speed of 155kmh^-1 in a time of 3 minutes 45 seconds. Calculate it's average acceleration over this time in units of ms^-2"

I know the velocity is 155 kmh^-1 and time is 225 seconds when changed from minutes to seconds and that the subject of the equation has to be A (acceleration) but I am unsure which equation to use to solve this.

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$\gamma=\frac{v_1-v_0}{t_1-t_0}$

with $t_1-t_0=3\times 60 +45$ s.

which gives

$\frac{ \frac{ 155000}{3600} }{ 225 }$ in $ms^{-2}$

$=\frac{1550}{36\times225}$

$=\frac{310}{36\times45}$

$=\frac{31}{18\times9}$

$=\frac{31}{162}=0.191 ms^{-2}$.