This probably isn't quite the question that one would expect on but i really want to know what exactly it was that i did wrong:

i was asked this question on e2020 (the e standing for education), and i was positive that i did all of the equations that i had to do right. i got the direction of r right on my first try (both ys add to zero, so the angle is automatically 0°), but i'm stumped on how i'm supposed to get 0.5n for the magnitude. if anybody knows the process that i missed, then let me know. i took a screencap of the question that this happened on for you to have a reference.