Guys... Like I said, it's impossible. A positive a value means that the parabola will open upwards. A postive b value will cause the slope of the graph to be positve at the y intercept. A positive c value will shift the graph verticaly so that it is raised up above y = 0 at the y intercept. So therefore, with only natural numbers, infact even with whole numbers, the zeros will both be at x values <= 0.
@Ladios, if some teacher is giving you this, tell him that it's a trick question because such a thing is impossible with what you described.
@Everyone else, Really think about it. If you find something that disproves my above point, please tell me.