rollercoaste starts at 12m high with a v of 1.2m/s and climbs to 9.0m Assume the track is frictionless...

a) how fast is it going when it reaches the low pointbetween the crestss.

b)how fast is it going at the top of the secondcrest (9.0m)

i have no idea which eq. to use..

I do not understand your diagram but use

Ke + PE = constant since no friction
so
(1/2) v^2 + g h = constant during your whole problem (mass does not matter, same in PE and KE)

get .5 v^2 and g h at the start, and as h goes down, .5 v^2 will go up by the same amount.