Given,
A(1,3,2),B(-2,8,0)&C(3,6,7)
And angle bisector of ∠BAC meets the line BC at D,
Now, plotting the diagram we get,

Now, finding
AC=2i^+3j^+5k^
⇒∣AC∣=4+9+25=38
And ∣AB∣=9+25+4=38
Using angle bisector theorem, we get
CDBD=ACAB
⇒BD:CD=1:1
Using section formula to find coordinates of D,
⇒D≡(1+21×3+1(−2),1+21×6+1×8,1+21×7+1×0)
⇒D≡(21,7,27)
⇒AD=−21i^+4j^+23k^
⇒AD=21(−i^+8j^+3k^)
Length of projection of AD on AC is given by,
⇒d=∣ACAD⋅AC∣
⇒d=23837