All circles are similar. This means that the ratio of the radius to the circumference of every circle is equal.
For a cut-out of a circle, the same ratio holds, as long as the angle of the circumference from the center of the circle is preserved.
From the image provided, we have that the angle is the same in both circles. Hence, we have that:
[tex]\frac{42}{18}=\frac{x}{13}[/tex]taking x as the circumference of the unknown