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˚