The 'a' determines if the V shape is upside down or not. If a > 0, then we have a V shape that isn't upside down. If a < 0, then the V shape is upside down as this diagram shows. In this case, a = -1.
The (h,k) is the vertex of the graph. For upside down V shaped graphs, the vertex is the highest point, which is (1, 3) in this case.