BUT t is in seconds and the 15 is mi/h, so we need to change the unit. In addition we will want x to be in feet, so we need to change miles to feet. There are 3600 s in 1 h, and there are 5280 feet in 1 mile:
So in the units we want.
Now, the balloon is at an altitude of 100 ft, so by the Pythagorean theorem the balloon is a distance of feet from the intersection.