Ok let me help you with this:
!n case a) Line both up head-to-tail in a straight line... that's the only way to get a sum 2F. In case b) sqrt(2) is the length of the hypotenuse in a 45 degree triangle, so the vectors must be at 90 degrees to each other
in the case of c) lined up head to tail, the only way to get 0 is if they point in opposite direction
