If foot of the perpendicular from a point \( P(a,b,0) \) on the line \( \dfrac{x-1}{2}=\dfrac{y-2}{1}=\dfrac{z-\alpha}{3} \) is \( A \) and mid-point of \( AP \) is \( \left(0,\dfrac{3}{4},-\dfrac{1}{4}\right) \), then the value of \( \left(a^2+b^2+\alpha^2\right) \) is -