Look at the picture.
We must use the Pythagorean theorem two times.
first time:
[tex]d^2=4^2+6^2\\\\d^2=16+36\\\\d^2=52\to d=\sqrt{52}[/tex]
second time:
[tex]AB^2=1^2+(\sqrt{52})^6\\\\AB^2=1+52\\\\AB^2=53\to AB=\sqrt{53}[/tex]
