Let r be the distance between the man and point Q. He applies a torque of
(r i ) × ((-800 N) j ) = ((-800 N) r ) k
pointing "into the page" with magnitude (800 N) r.
Meanwhile, the rope applies a torque of
((3.0 m) i ) × ((500 N) j ) = (1500 N•m) k
pointing out of the "page" with mag. 1500 N•m
The beam is held in place in a horizontal position, so the torques cancel and
(800 N) r = 1500 N•m
r = (1500 N•m)/(800 N) = 1.875 m ≈ 1.9 m