5cis120 degrees
Find the 3rd root

Finding roots for problems like this is really quite simple -- just remember that (R cis \theta)^n = R^n cis (n*\theta) . So here you have R = 5, \theta = 120 ,and n = 1/3. Also keep in mind that the angle \theta can also be written as \theta + 360, \theta + 720, etc. so in finding the nth root you will have multiple answers each varying by 360/n degrees from each other. Once you find one root, the other two can be found by adding multiples of 120 degrees. Can you take it from here?