We don't need any math for the first one, NSS
We can never get a negative answer out of a positive root
Thus....all the answers that have negative x values included are no good
So....by process of elimination....the last answer is correct !!!
Second one
We have this
0.75 ( 140) = s^2 / [ 30.25 ] multiply both sides by 30.25
(30.25) ( 0.75) (140) = s^2
Take the positive root of both sides
√ [ (30.25) ( 0.75) (140) ] = s ≈ 56 mph