Can someone please explain

The answer is 15.

Draw an imaginary line from A to C. You’ve just made a triangle. Using the Pythagorean Theory where a^2+b^2=c^2, we can find the distance.
One leg of your triangle is 9 and the other is 12. Plug those values into the formula above. It does not matter the order.

9^2 + 12^2 = c^2
81 + 144 = c^2
225=c^2
Sqrt225 = sqrtc^2
15 = c

Sqrt is short for . square root