A bag contains 10 red marbles, 15 yellow marbles, 5 green marbles, and 20 blue marbles. Two marbles are drawn from
bag.
Which expression represents the probability that one of the marbles is red and the other is blue?

 and vector B⃗ has a magnitude of 3.00 and is directed parallel to the positive y-axis. 

that means Vector OB (0, 3), so C=A – B means  vectOC = vect OA  -  vect OB =(0,-3)-(0,3)=(0,-6), so the magnitude of OA is sqrt((-6)^2)=6, so C=6 

and according to the information (as measured counterclockwise from the positive x-axis), so the answer is C = 6.00; θ = 90˚