If the area of a parallelogram is 15 cm 2 what is the area of the green triangle?

That would depend on the dimension of the green triage. If the triangle was formed by joining two opposite corners of the parallelogram then it would be half the area of the parallelogram.

Area of parallelogram = 15*2 = 30 square cm.

1/2 the area = 15 square cm.