Fabricating hood scoop and bad at math... any help?

Your "?" dimension is 9.96242942259"
Sorry I couldn't be more exact!

I think that only works if the angle is 90 degrees. It's not. I mocked it up in cardboard and it doesn't work. The angle is smaller than that, by a smidge.

I should have mentioned that my cardboard mock-up was originally made with 90* angles on the triangles and it caused a gap in the middle, from the sides to the hood. So I cut it from the bottom front corners to the back, making it a little shorter. Was just trying to see how it would look and didn't have exact dimensions. But definitely want to have measurements right before I start cutting sheet metal.