Get dz/dx=2x+2y *dy/dx.
2x+4y *dy/dx +6z *dz/dx=0
Two people stand together.
2dz/dx = 4x+4y * dy/dx = 4x-6z * dz/dx-2x
So I have to solve it.
dz/dx=x/( 1+3z)，
The substitution result is dy/dx = [x/(1+3z)-2x]/2y = x/(y+6yz)-x/y.