Answer:
2.3 kg
Explanation:
The plank is uniform, so the center of mass is in the middle, 2 m from the right end. That means it's 0.5 m from the saw horse on the right.
Sum of the moments about the right saw horse:
∑τ = Iα
m (9.8 m/s²) (1.5 m) − (7 kg) (9.8 m/s²) (0.5 m) = 0
m = 2.3 kg