I don't quite get your definition of heading. With a heading of 270 degrees point b is headed straight south isn't it? It means it must be somewhere on the positive y-axis to be going towards a, like at (0,10) or something.
If xX is horizontal and Y is vertical, then 10, 0 is 10 units to the right of 0,0 alnong the X axis. A 270 degree heading would be due West. And point straight to 0,0.

The problem itself is basic trigonometry: Determine the heading of the vector starting at b ending at a. That's the heading b must have to end up at a (travelling along a straight line).
Absoulely!