We can find the slant height ,s, as follows
12 = (1/2) *3 * s
24 = 3 * s
s = 8 cm
And since we have a 30 - 60 -90 right triangle the true height is (s/2) = 8 / 2 = 4 cm
So....the volume of the pyramid =
(base area * height) / 3 = (15 cm^2 * 4 cm) / 3 = 60 / 3 = 20 cm^3