Ta có : $\dfrac{x}{5}=\dfrac{y}{3}=\dfrac{z}{7}=\dfrac{3x+2y-5z}{3.5+2.3-5.7}=\dfrac{12,24}{-14}=\dfrac{-153}{175}$
$\Rightarrow x=5.\dfrac{-153}{175}=\dfrac{-153}{35}$
$y=3.\dfrac{-153}{175}=\dfrac{-459}{175}$
$z=7.\dfrac{-153}{175}=\dfrac{-153}{25}$
$\Rightarrow 8x+y+3z=8.\dfrac{-153}{35}+\dfrac{-459}{175}+3.\dfrac{-459}{175}=\dfrac{-7956}{175}$