Question 1044589
Hi there,
Consider that in folding you have 
created a right angled triangle
with a side of 12" and a side of
9".
Using Pythagoras Theorem.
12^2 + 9^2 = crease^2
144 + 81 = crease^2
crease size = √225
crease size = 15"
Hope this helps. :-)