ok this is going to have a long explanation

use the pythagorean theorem to solve this (a2+b2 = c2)

imagine the line as the hypotenuse of a triangle. we already have a2, which is 12, but we don’t have b2 or c2. c2 is the diagonal

to find b2, apply the pythagorean theorem to the 8in and 6in

8^2 + 6^2 = c^2

reducing this makes it 64+36=c^2

add the left side together to get 100=c^2

square root both sides to get 10=c

now we know that the bottom of the “triangle” is 10

now, to find the length of the line, apply the 12 in and 10in we just found into the pythagorean theorem.

12^2+10^2=c^2

reduce it to get 144+100=c^2

add to get 244=c^2

square root both sides to get 15.62=c

the length of the diagonal between A and B should be around 15.62

hope this helped!!