Acar moving at 18 m/s decelerates at -- 2.5 m/s" as it approaches a stop sign.
a. how long does it take the car to come to a complete stop?

The car takes 7.2 seconds to come to a complete stop when moving aExplanation:

t 18 m/s decelerates at - 2.5 m/.

From the given statements, we know

Initial Velocity (u) = 18 m/s

Final Velocity (v) = 0 m/s

Acceleration (a) = - 2.5 m/

Applying the 1st Equation of Motion  

v = u + at

and substituting the values, we get

                      t=

                          =     = 7.2 seconds

d) the number of neutrons plus the number of protons