use conversion of energy. mgh+.5mv^2 initial = final. at the top there's no kinetic energy, at the bottom theres no potential. so mgh at the top=.5mv^2 at the bottom.
mgh at the top = 40 (9.8)(12)
not sure what you mean by 10 miles, do you mean 10 mph or 10 meters/second. if it isnt in SI units, do the proper conversions and put it in m/s
.5mv^2=.5 (40)(final velocity)