Question

I just need to know what formula I need to use to get started on this equation: A man that was shot out of a cannon and the mean is 140 feet, stand. dev. is 10, landing net is 28 feet long. To maximize the peron's probability of landing on the net, how far away from the cannon should he be position the nearest edge of the net?
I know the answer is 126 but I cannot figure out how to get it. Could some get me started on this? Thanks!