The car has an initial velocity [tex]v_0[/tex] of 23 m/s and a final velocity [tex]v[/tex] of 0 m/s. Recall that for constant acceleration,
[tex]v=v_0+at[/tex]
The car stops in 7 s, so the acceleration is
[tex]0\,\dfrac{\mathrm m}{\mathrm s}=23\,\dfrac{\mathrm m}{\mathrm s}+a(7\,\mathrm s)[/tex]
[tex]\implies a\approx-3.29\,\dfrac{\mathrm m}{\mathrm s^2}[/tex]
