The product of the lengths of the parts of the chord is equal to the product of the lengths of the parts of the chord that divides it.
(7 cm)*(8 cm) = n*(4 cm)
(7*8 cm²)/(4 cm) = n
14 cm = n
(7 cm)*(8 cm) = n*(4 cm)
(7*8 cm²)/(4 cm) = n
14 cm = n