2m^4+m^2n-3n^2=(2m^2+3n)(m^2-n)
已知：m^2-n=45,4m^2+6n+2/3=0
所以：m^2-n=45,2m^2+3n=-1/3
2m^4+m^2n-3n^2=(2m^2+3n)(m^2-n)=-1/3*45=-15