Answer:
D. 12
Step-by-step explanation
You know this is an isoceles triangle because the two base angles on congruent. That means the two sides are equal. So, 3x = x+8. If you plug in each of the answer choices, you'll notice that 4 fits the equation. If 4 equals x, 4 times 3 equals 12 and on the other side, 4+8 also equals 12. Therefore, to find the length of MN, you would just need to do 4+8 which equals 12