Since the angle at the top is the same, then we can see the triangles are as well. So if we can find the rate the two congruent sides went down, then we can see how to solve for DE.
First lets see how much 8 is of 12. To do that, make a fractions, then reduce as low as possible.
8/12=4/6=2/3
8 is 2/3 of 12, so now all we have to do is find 2/3 of 9
9*(2/3) = 6
So the answer would be A.6
Hope this helps!