3the other diagonal is let x+7y+k=0 whare k is a constant..as the diagonals are perpendicular in a square
as The point (–4, 5) is the vertex of a square and as it doesnot satisfy 7x – y + 8 = 0.
so it must pass through x+7y+k=0
thus k=-31
hence The equation of the other diagonal is (C) 7y + x = 31