Hey if someone could help me on some homework I have i'd really appreciate it. I'm stuck on this problem.

I know you have to add the vectors to get the location. like (3,x) +(-5,-12), but I'm stuck from there. The last bit has no coordinates (x,0). For the first vector do I need to use Pythagorean theorem to solve for y? By adding these coordinates I would get the location, but how do I get the magnitude? Do I add 10, 4 and 16?