The car's AVERAGE speed is (21/2)=10.5 m/s. (Zero at the beginning, 21 at the end.). In order to cover 50m at an average speed of 10.5 m/s, it takes (50/10.5)=4.76 seconds. It took 4.76 seconds to accelerate to 21 m/s^2, so the average acceleration is (21/4.76)=4.2 seconds. You probably had a formula in class that would take you to 4.4, but this is the way I think it through without a lot of formulas.
