This is a proportion problem because the problem stated that the two team pennants are similar.
Pennant 1 = base = 18 in ; length = 9 inches
Pennant 2 = base = 6 in ; length = x ?
9/18 = x/6
9*6 = 18x
54 = 18x
54/18 = x
3 = x
The length of the side of the smaller pennant is 3 inches.