x=a代入方程：a^2-3a-2=0,
两边乘以a:a^3-3a^2-2a=0 ,得：a^3=3a^2+2a
故a^3-2a^2-5a+3
=3a^2+2a-2a^2-5a+3
=a^2-3a+3
=a^2-3a-2+5
=0+5
=5