If the feet of the perpendiculars drawn from the point
\[
(3,4,5)
\]
to the \(X\)-, \(Y\)- and \(Z\)-coordinate axes are \(A,B,C\) respectively and the angle between \(AB\) and \(AC\) is
\[
\cos^{-1}\left(\frac{9}{a}\right),
\]
then the value of \(a\) is: