Given the vertex -1/2,3 and the directrix x=-13/24, what are the values for a, h, and k in the vertex form of the parabola?
a) a = 1/6, h = -1/2, k = 3
b) a = 1/3, h = -1/2, k = 3
c) a = 1/6, h = -1/2, k = -3
d) a = 1/3, h = -1/2, k = -3