Triangle ACD is congruent (identical to) Triangle BCD;
They both have the common side, CD, and have sides of common length, AC and CB as they are both radii;
As AC = CB, the angles CAB and CBA are equal also;
The fact triangles ACD and BCD are congruent means AD = DB;
We can use Pythagoras to find DB using CD and the radius:
DB² = r² - CD²
DB² = (51)² - (24)²
DB² = 2601 - 576
DB² = 2025
DB = 45mm
Since AD = DB, AB = 2(DB) = 2(AD) so:
AB = 2(45) = 90mm