The question relies on conservation of energy
2 Energy equations:
Kinetic Energy = 1/2mv^2
Gravitational potential energy = mgh
Initial energy = final energy
1/2m(v0)^2 = mgh + 1/2m(vf)^2
Divide mass out to save time
1/2(v0)^2 = gh + 1/2(vf)^2
PLug in what we know:
v0 = 39 m/s
h = 21 meters
1/2(39)^2 = 9.8(21) + 1/2(vf)^2
1/2(39)^2 - 9.8(21) = 1/2(vf)^2
554.7 = 1/2(vf)^2
1109.4 = (vf)^2
vf = 33.3076 m/s