Here are a few tips that will help you get it right(and some points even if you get the answer wrong ):
1)Draw a picture. This will not only help you understand the problem better, but it will also let the grader see what you are doing and how you understand the problem.
2)Make a table. Have an x column and a y column. Under the the x colunm, right the x-component of each vector(even if it is zero). Do the same thing with the y-components in the y colunm. Draw a line under your last row and make a total row. Have a x total and a y total. Making a table not only helps you keep track of your components and see mistakes easier, it makes your work more organized and the grader will be able to find partial credit easier.
With your problem, the table should be pretty simple since your vectors have either only an x component or only a y component.