An isosceles triangle have two equal sides. In this case, it can be either 2 or 12. One of them is correct, and we can use the existence theorem of a triangle. That is:
|L1 - L2| < L3 < L1 + L2
Here, we can choose: L1 = 2 and L2 = 12 (the order doesn't matter)
|2 - 12| < L3 < 2 + 12
10 < L3 < 14
So the remaining side must be greater than 10 and smaller than 14.
The correct lenght of L3 is 12
Option C is the correct answer
