We can use the Pythagorean theorem to solve this.
We can first draw two more lines to complete the triangle.
We then have two more lines, one with a length of 5, and another with a length of 7.
The Pythagorean theorem states that a^2 + b^2 = c^2.
7^2 is 49, and 5^2 is 25.
Add those together and you get 74.
We then square 74, and get 8.602, which we can round to 8.6.
The length of segment AB is 8.6.
Hope this helps!