You find the prime factorization by breaking the number down into other numbers that are prime. Start by breaking up 312 into 39 * 8. 39 breaks up into 3 * 13, and 8 breaks up into 4 * 2 which breaks up into 2 * 2. So the prime factorization of 312 is 3 * 13 * 2 * 2 * 2 or . When you multiply those together you'll get 312.
Because it is an isosceles triangle, the two longer sides are the same length. Therefore multiply 6.3 times 2. To find the length of the base subtract 12.6 from 15.7 to get 3.1 cm.