Official solution
\[\mathrm{B}=(2 \lambda+7,-3 \lambda-2,6 \lambda+11)\]
Point B lies on \[\frac{x-6}{1}=\frac{y-4}{0}=\frac{z-8}{3}\]
\[\frac{2 \lambda+7-6}{1}=\frac{-3 \lambda-2-4}{0}=\frac{6 \lambda+11-8}{3}\]
\[-3 \lambda-6=0\]
\[\lambda=-2\]
\[\mathrm{B} \Rightarrow(3,4,-1)\]
\[\mathrm{AB}=\sqrt{(7-3)^2+(4+2)^2+(11+1)^2}\]
\[=\sqrt{16+36+144}\]
\[=\sqrt{196}=14\]