The simple answer to this is that more people would vote if voting were required by law. But that is most likely not why you are being asked this question. Instead, you are being asked if this would be a good thing. Many people feel that the fact that we have such low turnout in American elections is a bad thing. So, would it be a good thing if voting were required and, therefore, more people voted?
This is a matter of opinion. My own opinion is that things would not be better. My main reason for saying this is that we would not get a better class of voters. Instead, we would have more voters, but many of the people voting would not care much one way or the other and would be likely to cast their votes irresponsibly.
While we do want more people to vote, we only really want them to vote if they care enough to inform themselves. A person who is forced by law to vote will simply tend to vote on a whim. This is not good for democracy. I do not think that it is good for the government to infringe on people’s liberties when the only outcome would be to have people voting without putting any real thought into their decisions.