So if the larger pennant is 18”x 9” x 9”
and the smaller one is about the same in similar size, with the bass measuring 6”..
I would take the bases of both and divide them 6” smaller base goes in to the 18” larger base 3 times so I take the sides 9” and see what goes into it 3 times 9 divided by 3 is 3 .. so the sides would be 3 making the smaller triangle 6x3x3