The weight 2N of the bar acts at its midpoint D. Let C be the position of the knife edge & W be the weight suspended at the end A which keeps the bar horizontal. Taking moments about C
W × AC = 2 × OD
W × 1 = 2 × 2.5
W = \(\frac{2\times2.5}{1}\) = 5N
To keep the bar horizontal the weight to be suspended is 5N.