x=m
则m²+3m+1=0
m²=-3m-1
所以m³=m*m²
=m(-3m-1)
=-3m²-m
=-3(-3m-1)-m
=8m+3
且m+n==-3
所以原式=8m+3+8m+20
=8(m+n)+23
=-1