6. (8th grade) The lengths of two sides of a triangle are 7 cm and 11 cm, respectively. Find the length of the third side if it is expressed as a prime number of centimeters.
6. The desired side can have a length greater than 4 and less than 18 cm, i.e., 5 cm, 7 cm, 11 cm, 13 cm, 17 cm.