I'm guessing that you want to calculate the middle horizontal line.
The upper left triangle on the left is a 45 45 90 triangle.
The hypotenuse is 12 and so the leg is 12 * (1/square root of 2)
leg = 12 *
0.7071067812
leg =
8.4852813742
So that means the line =
16.9705627485
