Diameter = 26 cm, Radius = 13 cm
In the circle, draw a chord and 2 radius' to get an isosceles triangle. You'll get something like this.
The distance were looking for is OM. Now focus on triangle OMB. It is a right angled triangle where OB = 13 cm and MB = 12 cm.
Using the pythagoras theorem,
OM² = OB² - MB²
OM² = OB² - MB²
OM² = 13² - 12²
OM² = 25
OM =5