How to post the questions on yahoo answers with fonts?

You can't. Computers are stupid things- they don't actually understand things like superscripts and subscripts. We have to write programs that interpret those for us. So for instance, web pages use x² to describe what you're looking for. Yahoo Answers, for security reasons and to make it easier for people who don't realize that you can't normally put < and > in a webpage (you have to write them as < and >) only lets you write plain text.