A combat jet is flying at 750 mph, at 40,000 feet above the ground. It needs to descend all the way to 100 meters above the ground in order to engage the target, which is an oil depot several miles ahead. It descends along a diagonal, maintaining a constant speed of 750 mph along the diagonal. Ground radar at the oil depot shows the jet approaching along the horizontal at 738 mph. How fast is the jet descending vertically, in mph?