Physics
posted by Hannah .
1) A jetliner, traveling northward, is landing with a speed of 69.3 m/s. Once the jet touches down, it has 787 m of runway in which to reduce its speed to 13.6 m/s. Compute the average acceleration (magnitude and direction) of the plane during landing (take the direction of the plane's motion as positive).
I know that the formula for average acceleration is change in velocity / elasped time.
To get the change in velocity I did 69.313.6. I am not sure what the elasped time is.

a ={v(final)²  v(initial)²}/2•s =
=(13.6²  69.3²)/2•787=  2.93 m/s²