The answer is the square root of 101.75
Assuming that the sandwich Is a square, after cutting it diagonally, it turns into 2 triangles. To look for a missing length of a triangle, you use Pythagorean’s theorem a^2+b^2=c^2
Since c is 12 and a is 6.5 you just square 12 and square 6.5
144 42.25
Subtract it
101.75
You then have to find the square root of it. You can leave it as it is as √101.75 or just solve for it and round it to 10.09