Determine for the two vectors:

$x=ai+bj+ck$ and $y=di+ej+k$

where

$a=2\mathrm{.}9,b=4\mathrm{.}8,c=-3\mathrm{.}5,d=3\mathrm{.}5$

and $e=5$

the value of $$ such that $x$ and $y$ are perpendicular. Give your answer to three decimal places.