This is because both sides are equal. And while this isn't the best explanation, my reasoning is because if you were to cut it in two and flip one side over the other side they would be equal. Also since all sides are not equal all are not the same angle. Apologies if this doesn't make sense, I am not good and explaining stuff. Best of luck on the assignment.
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.
