drawing a line from the center of the circle to the mid point of the chord forms a right triangle with the hypotenuse being the radius 5cm and half the chord being 4cm. you need to find the third side use Pythagorus.

h^2 = a^2 + b^2

289 = 16 + b^2

b^2=273

b=sqrt273

b=16.523

The length of cord from the centre is 16.5223 cm