In coordinate space, the distance between point $\mathrm { P } ( 0,3,0 )$ and point $\mathrm { A } ( - 1,1 , a )$ is 2 times the distance between point P and point $\mathrm { B } ( 1,2 , - 1 )$. What is the value of the positive number $a$? [2 points]\\
(1) $\sqrt { 7 }$\\
(2) $\sqrt { 6 }$\\
(3) $\sqrt { 5 }$\\
(4) 2\\
(5) $\sqrt { 3 }$