The Moon Giant Killer

You're the only person stationed at a remote lunar outpost. Your job is to prevent any of those gargantuan moon giants from eating you. Fortunately, these giants are few and far between: just a handful over all of Mare Imbrium. Unfortunately, you have only one harpoon at your outpost. It has already been loaded into the launcher, which, when you push the button, will fire it at 30 meters per second at an angle of 30 degrees from the horizontal. Suddenly, you see a moon giant lumbering along the desolate landscape. At a distance of 858 meters, he stops and realizes you're there. Famished, he accelerates toward you -along a straight line path- at 4 meters per second squared.. You know that moon giants have only one vulnerable spot. The giant can only be killed if the harpoon strikes its naval. This giant's naval is 25 meters from the ground. So, too, is the harpoon launcher. Assume the giant is too clumsy to attempt any evasive maneuvers. You should fire your harpoon after the giant has run how far from his starting point? (Don't worry about air resistance - there isn't any; Use 1.7 meters per second squared for the moon's surface gravity value.)