This is a conservation of mechanical energy question
Suppose the minimum velocity is v
Then mechanical energy at top of 15m hill is 0.5 m v2 + mgh = 0.5 mv2 + 15mg
g = 9.81 m/s/s (acceleration due to gravity).
All of this (assuming no loss due to friction) is converted to potential energy at top of 29m hill
So 0.5 m v2 + 15mg = 29mg
You can solve this for v