求满足方程组2x-y-4m=0 14x-3y=20 中的y值是x值的3倍的m的值,并求xy/x+y的值

y=3x
代入14x-3y=20
14x-9x=20
5x=20
x=4,y=3x=12
代入2x-y-4m=0
8-12=4m
m=-1
xy/(x+y)
=4*12/(4+12)
=3