Draw the argand diagram ..
the two given vertices are (-1,0) and (1/2, root3/2).... so the two position vectors will be at angle 60 and 180 with positive x axis.. hence the third vector must be directed at -60 degree with the positive x axis, so that the three vectors are at 120 degree with each other and hence form a equilateral triangle...
while solving objective question, always try to draw the diagram if you can... there may be some simple shortcut for the problem.