∵ a/b=d/c=f/e=3
∴a=3b,d=3c,f=3e
∴(a+2d-3f)/(b+2c-3e)
=(3b+2*3c-3*3e)/(b+2c-3e)
=3(b+2c-3e)/(b+2c-3e)
=3