Answer
Zack should throw the book directly outward along the path shown by arrow 2.
Work Step by Step
The two cars are moving with the same speed and the book is also moving with this same speed. If Zack wants to throw his book into the other car, there is no difference in this situation compared with the situation when both cars are at rest, because the relative speed between the two cars is zero. Therefore, from Zack's point of view, he should throw the book directly outward along the path shown by arrow 2. Then the book should go in the window of the other car.
