this is weird tho but since it's regular that means it's equiangular/equilateral. So every angle is 60°.
The 8 is half of of the triangle (I think??) so you'd have to turn it to 16 so it'll be full. It'll then be an altitude so I'll make a right angle.
Now you've got two congruent triangles, both of them are 30-60-90 special right triangles so you can use sohcahtoa.
Use the 60° angle to find the hypotenuse
sin(60) = 16/h
(h)sin(60) = 16
/sin(60) on both sides
18.48 is the answer
If you want that as a radical then its somewhere around 3√2
That means every side is 3√2 because it's regular so do 3√2 x 3√2 which i think is 12 ??? Maybe???? Idk sorry