Response Time Activity

In the response time activity my delta x was 42 centimeters/0.42 meters. The acceleration was -9.8m/s^2 and the initial velocity was 0m/s. To calculate the time I used the following procedure.
delta x=Vi delta t + 1/2 a(delta t)^2
-0.42=0(delta t) + 1/2 (-9.8)(delta t)^2
-0.42=0(delta t) + (-4.9)(delta t)^2
-0.42=-4.9(delta t)^2
-0.42/-4.9=detla t^2
0.292s=delta t