Hi there!
There are two torques acting on the system:
τ (Arthur) = RW = R(790N)
τ (Violet) = RW = 4(500N) = 2000N
∑τ = 0 = τ(Violet) - τ(Arthur)
Thus:
τ(Violet) = τ(Arthur)
2000 = 790R
R ≈ 2.53m
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