Your answer would be 7.94 units.
First we must note that a perpendicular bisector halves the side it is bisecting, so FA = FB. This then allows us to say that FA = 9, as 18 ÷ 2 = 9.
Now that we know FA = 90, we can see the right triangle with a hypotenuse AD, which is 12, and a leg FA, which is 9.
This then allows us to use Pythagoras's Theorem to find a leg of a right triangle, c² - b² = a², so we can do:
12² - 9² = 144 - 81 = 63
√63 = 7.93725, which is 7.94 to the nearest hundredth.
I hope this helps!
