(mx+1)(-3x+4)
=-3mx^2+4mx-3x+4
=-3mx^2+(4m-3)x+4
因为不含x项
所以:(4m-3)x-3mx^2=0
4mx-3x-3mx^2=0
m(4x-3x^2)=3x
m=3/4-3x