The lamp descends with the elevator so its acceleration is also 2.9 m/s². Take downward to be the positive direction. Then the net force on the lamp is
∑ F = w - t = ma
where w = mg = weight of lamp, t = tension in the cord, m = mass of lamp, and a = acceleration. Then
mg - t = ma
mg - ma = t
m (g - a) = t
m = t / (g - a)
so that the lamp's mass is
m = (52 N) / (9.8 m/s² - 2.9 m/s²) ≈ 7.5 kg
