Based on the information given, we can use the Pythagorean theorem to determine the distance traveled by the northbound car.
Let's denote the distance traveled by the northbound car as 'x' kilometers.
According to the Pythagorean theorem, in a right triangle, the square of the hypotenuse (the straight-line distance between the two cars) is equal to the sum of the squares of the other two sides (the distances traveled by each car).
In this case, the northbound car's distance is 'x' kilometers and the eastbound car's distance is 5 kilometers.
So we have the equation:
x^2 + 5^2 = 13^2
Simplifying, we get:
x^2 + 25 = 169
Subtracting 25 from both sides, we get:
x^2 = 144
Taking the square root of both sides, we get:
x = 12
So the northbound car has traveled 12 kilometers.