As per the question,
2x + 4y + 3z = 5 and x + 2y + λz = 12 are perpendicular.
We can conclude that their normals will also be perpendicular.
n1 = (2, 4, 3) and n2 = (1, 2,**λ) so, n1 . n2 = 0**
2x1 + 4x2 + 3x**λ = 0 2 + 8 + 3**λ = 0λ = -10/3