I'm assuming this is homework... otherwise there's no sane reason you'd want to do it. Therefore I'll just give hints...
If performance isn't terribly important, consider that x * 3 = x + x + x... think about using a loop.
If performance is important but you know that one of the numbers will be small, loop on the smaller number.
If performance is important and both numbers could be large, you'll need to think about bit-twiddling. Remember that x * 2 is x << 1, and go from there.
