When acceleration is constant, average and instantaneous acceleration are the same, so
[tex]a=\dfrac{\Delta v}{\Delta t}=\dfrac{v-v_0}t\implies t=\dfrac{v-v_0}a[/tex]
Then
[tex]t=\dfrac{12\,\frac{\mathrm m}{\mathrm s}-7.0\,\frac{\mathrm m}{\mathrm s}}{2.5\,\frac{\mathrm m}{\mathrm s^2}}[/tex]
[tex]\implies t=2.0\,\mathrm s[/tex]
