=How to say "No" politely in English=
A : Hey Taro, I was going to visit my client with John today, but he is off sick. Actually I need someone to visit my client together at 11am, could you come with me?
B : I wish I could, but I'm going to attend a meeting in ten minutes and I am preparing for it right now.
A : Ok, I guess I'm going to handle it myself.
B : I could visit there after my meeting, maybe around 12 o'clock, if you still need it.
A : Really? That would be great. Thank you Taro,