you have vector A and vector B subtract them and whatever you get let's call it D, so you will have D+3C=0=>-D/3=C. Remember keep D in x and y component and then divide it by 3.
if A, B, C are vectors from origin to the points A, B, C show that the following is perpendicular to the plane ABC: (AxB) + (BxC) + (CxA)
I am having trouble setting up the problem. I can't understand the vector A, B, C. is vector A = Ai + Aj + Ak? and so on?