Kinematics-Boulder Falling Problem

1. The problem statement, all variables and given/known data
I’m not sure if this is the right place to post this but I need help understanding this question…
Discounting air friction, approximately how far will the boulder have fallen in 2 seconds?

a. 20m
b. 45m
c. 30m
d. 90m

2. Relevant equations

x-x_0 = v_0t + 1/2at^2

3. The attempt at a solution
Now I know this is just a plug and chug equation and the answer is 45m (B). My question is more in understanding the physics behind using this equation….
Originally i used a=Δv/Δt then plugged my found velocity into v=d/t but that gives me 40m, which is not an option.
How do I know when to use which?

Thanks for any help in clarifying this for me! 😀

http://ift.tt/1vUPkBL

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