Always start at the origin
Think (left or right, then up or down...plot a point)
Negative numbers on the X axis ask you to move left
Negative numbers on the Y axis ask you to move down
I reached point B by moving right 30 from the origin and then moving down to the -10 on the Y axis. I plotted a point at the intersection of the two line segment found at (30, -10).
Best of luck!