Look up the theorem on intersecting chords.
From the circumference, find the radius. From this, and the fact that the line bisects 426mm below center, find the angle that the line segment makes at center. Get it?
Salahuddin
Maths online
I assume that "426 mm down" means that the line from the center of the circle is perpendicular to the chord. Those lines together with the radii from the center of the circle to the ends of the chord give you two right triangles. Each has hypotenuse 7660 and "near side" of length 426 so that . The central angle, then, is and the arclength is .