An elevator is moving upward at 0.90 m/s when it experiences an acceleration 0.40 m/s2 downward, over a distance of 0.64 m. What will its final velocity be?

Vf^2 = Vo^2 + 2a*d.

Vf^2 = 0.9^2 + (-0.8)*0.64 = 0.298
Vf = 0.546 m/s.