This is an isosceles triangle with a perimeter of 12+ 18=30 and a difference of 18- 12=6. There are two answers:

(1) If the bottom edge is less than the waist length, the two waists of AB and AC are (30+6)÷3= 12, and the bottom edge BC is 12-6=6.

(2) If the bottom edge is longer than the waist length, the two waists, namely AB and AC, are (30-6)÷3=8, and the bottom edge BC is 8+6= 14.