The easiest way to find this is to actually graph the line. We know that one of the points is at (6, 0), so we can make our first point there. Then, we should just follow our slope until our x-value is equal to 15. The y-value when the x-value is 15 is the correct answer.
Below, I have attached a graph that should help you understand. The highlighted numbers are the numbers at the intersection. The red circle shows the intersection point. The blue circle shows the original point we started with, and the purple dots show our slope. Using the graph, we were able to find that y=6.