Just use algebra to rearrange the terms so for s=1/2at^2 and solving for a we get 2s=at^2 so 2s/t^2=a solving s=1/2at^2 for t gives 2s=at^2 which leads to 2s/a=t^2 so t=sqrt(2s/a)
filling in the numbers for 3 since a = 2s/t^2 we get a= (2(20.5m))/5s^2 =1.64m/s^2 the correct units in the answer indicate a possible correct answer since the units of acceleration are m/s^2.
4. using the value for a from question 3 and using 42.5 meters for s and solving for t we get sqrt((2s)/a) = sqrt((2*42.5m)/1.64m/s^2)= approximately 7.2s you can see the m's cancel and you are left s^2 as the units and the sqrt(s^2) = s so again the correct units indicate that 7.2s is probably correct. (These answers ARE correct though). checking the units gives a kinda quick assurance you are on the right path.
