5m-n=2(3m+2n)

5m-n=6m+4n

-m=5n

m=-5n

(m³+2m²n)/(n³-mn²)=m²(m+2n)/(n²(n-m))=25n²(-5n+2n)/(n²(n-(-5n))=

=25(-3n)/(6n)=-75/6=-12.5