physic vector

Lets see...

R=B+C

R=C

Do you see the answer?

From what I remember, magnitude is found by Pythagoras.
So R has mag =5 and if it is in the positive y direction, R=5j

So 5j=B+ 3i +4j hence B=-3i +j which gives mag= sqrt(10)
and direction will be given by arctan (1/-3) as the angle to the x axis