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
because the angles at the bottom of each segment are equal, the length of each segment is equal as well, so set you equation as 3x=x+8, subtract x from 3x then divide 8 by 2 which equals 4. (x=4) then plug in the value of x into x+8 and you have 12
