[m]\vec{d}\cdot\vec{b}=0[/m] ⇒ [m]x_{d}\cdot 4(-3)+y_{d}\cdot 1+z_{d}\cdot 2=0[/m]
[m]\sqrt{x^2_{d}+y^2_{d}+z^2_{d}}=1[/m]
Решаем систему трех уравнений:
[m]\left\{\begin {matrix} x_{d}\cdot 4+y_{d}\cdot 3+z_{d}\cdot 1=0\\x_{d}\cdot 4(-3)+y_{d}\cdot 1+z_{d}\cdot 2=0\\x^2_{d}+y^2_{d}+z^2_{d}\end {matrix}\right.[/m]