Question

A car traveling at 20.0 m/s starts to decelerate at a constant rate. It comes to a stop in 7.5 seconds. What is the cars acceleration

Answer 1

$$a=\dfrac{v-v_0}{t}=\dfrac{0-20\ \dfrac{m}{s}}{7.5\ s}=-2.67\ \dfrac{m}{s^2}.$$

The sign minus means that the car decelerates.