Now, they say at a certain instant, t sub zero. So let's draw that instant. So the first car is a distance x of t sub zero of 0.5 kilometers. So this distance right over here, let's just call this x of t, and let's call this distance right over here y of t. Now, how does the distance between the cars relate to x of t and y of t? Well, we could just use the distance formula, which is essentially just the Pythagorean theorem, to say, well, the distance between the cars would be the hypotenuse of this right triangle. Remember, they're traveling from perpendicular directions, so that's a right triangle there. So this distance right over here would be x of t squared plus y of t squared and the square root of that, and that's just the Pythagorean theorem right over here.