AC is the hypotenuse of a right triangle with legs 3 and 4.
The radius of circle C is 5.
_____ Surely you've heard of a 3-4-5 right triangle. If not, you can compute the hypotenuse from the Pythagorean theorem. .. AC = √(3^2 +4^2) = √(9 +16) = √25 = 5