Actually your question is a bit vague and lacks critical detail. Part 2 can be easily found in lots of places and even on Server Fault with a simple search and a bit of rummaging through the results.
The first part lacks critical detail.
- How are you trying to connect from the client to ServerB:3306 via server A presumably?
- Have you enabled routing on server A ?
- Have you tried to connect from server A to Server B 3306 was it successful or not?
- When you try to connect from the client to ServerB:3306 what error messages do you see ?
- Have you tried using something like tcpdump to trace the connection through the network ?
People have just downvoted your question based on what they can see and that's not great.
Downvoting isn't an unfriendly action it's a reaction to poor quality.
You are way overreacting to this.