ANSWER
[tex]AB=12\text{ }mm[/tex]
EXPLANATION
We want to find the length of AB.
The length of AB represents the radius of the circle since it is the distance from the center of the circle to the circumference.
We are given that the diameter of the circle is 24 mm. The radius of a circle is half of its diameter, hence, the radius of the circle is:
[tex]\begin{gathered} r=\frac{24}{2} \\ r=12\text{ }mm \end{gathered}[/tex]
Therefore, the length of AB is:
[tex]AB=12\text{ }mm[/tex]
That is the solution.
