Since the triangle is a right angled, to find one side we should use the pythagoras theorem.
The longed side of the triangle is called the hypotenuse.
In equation form: x² + y² = z²
Therefore: 12² + x² = 13²
114+x²= 169
x²= 169-114
x²= 55
x = 7.41
Therefore the length of AC = 7.41