To find for the velocity, we use the Kinetic Energy formula:
KE = (1/2)m*v^2
Where
KE = Kinetic Energy, 720 lb ft
m = 0.0010 slugs
v = unknown
Rearranging the equation and solving for v:
v^2 = 2(KE)/m
v^2 = 2(720)/0.001 = 1,440,000
v = 1200 ft/s